Add check for zero entry velocity

[tonyluken]

TinyG hard alarms and reports this error: {er:{fb:440.21,st:31,msg:"Move time is NAN"}}
on the last line of this G code snippet:

G1 X52.2955 Y48.9013   F60.68 
M400 
M114 
M201.3 X38884 Y48990
G1 X52.2960 Y48.9019   F12.30 

with this TinyG Configuration:

[fb]  firmware build            440.21
[fv]  firmware version            0.97
[hp]  hardware platform           1.00
[hv]  hardware version            8.00
[id]  TinyG ID                    7X2109-JGS
[ja]  junction acceleration 2000000 mm
[ct]  chordal tolerance           0.0100 mm
[sl]  soft limit enable           0
[st]  switch type                 0 [0=NO,1=NC]
[mt]  motor idle timeout    1000000.00 Sec
[ej]  enable json mode            0 [0=text,1=JSON]
[jv]  json verbosity              0 [0=silent,1=footer,2=messages,3=configs,4=linenum,5=verbose]
[js]  json serialize style        0 [0=relaxed,1=strict]
[tv]  text verbosity              1 [0=silent,1=verbose]
[qv]  queue report verbosity      0 [0=off,1=single,2=triple]
[sv]  status report verbosity     0 [0=off,1=filtered,2=verbose]
[si]  status interval    4000000000 ms
[ec]  expand LF to CRLF on TX     0 [0=off,1=on]
[ee]  enable echo                 0 [0=off,1=on]
[ex]  enable flow control         2 [0=off,1=XON/XOFF, 2=RTS/CTS]
[baud] USB baud rate              5 [1=9600,2=19200,3=38400,4=57600,5=115200,6=230400]
[net] network mode                0 [0=master]
[gpl] default gcode plane         0 [0=G17,1=G18,2=G19]
[gun] default gcode units mode    1 [0=G20,1=G21]
[gco] default gcode coord system  1 [1-6 (G54-G59)]
[gpa] default gcode path control  0 [0=G61,1=G61.1,2=G64]
[gdi] default gcode distance mode 0 [0=G90,1=G91]
[1ma] m1 map to axis              0 [0=X,1=Y,2=Z...]
[1sa] m1 step angle               0.900 deg
[1tr] m1 travel per revolution   39.9286 mm
[1mi] m1 microsteps               8 [1,2,4,8]
[1po] m1 polarity                 0 [0=normal,1=reverse]
[1pm] m1 power management         2 [0=disabled,1=always on,2=in cycle,3=when moving]
[2ma] m2 map to axis              1 [0=X,1=Y,2=Z...]
[2sa] m2 step angle               0.900 deg
[2tr] m2 travel per revolution   39.9514 mm
[2mi] m2 microsteps               8 [1,2,4,8]
[2po] m2 polarity                 0 [0=normal,1=reverse]
[2pm] m2 power management         2 [0=disabled,1=always on,2=in cycle,3=when moving]
[3ma] m3 map to axis              2 [0=X,1=Y,2=Z...]
[3sa] m3 step angle               1.800 deg
[3tr] m3 travel per revolution    8.0000 mm
[3mi] m3 microsteps               8 [1,2,4,8]
[3po] m3 polarity                 1 [0=normal,1=reverse]
[3pm] m3 power management         2 [0=disabled,1=always on,2=in cycle,3=when moving]
[4ma] m4 map to axis              3 [0=X,1=Y,2=Z...]
[4sa] m4 step angle               0.900 deg
[4tr] m4 travel per revolution  160.0000 mm
[4mi] m4 microsteps               8 [1,2,4,8]
[4po] m4 polarity                 0 [0=normal,1=reverse]
[4pm] m4 power management         2 [0=disabled,1=always on,2=in cycle,3=when moving]
[xam] x axis mode                 1 [standard]
[xvm] x velocity maximum      10000 mm/min
[xfr] x feedrate maximum      10000 mm/min
[xtn] x travel minimum            0.000 mm
[xtm] x travel maximum          600.000 mm
[xjm] x jerk maximum         540000 mm/min^3 * 1 million
[xjh] x jerk homing            2000 mm/min^3 * 1 million
[xjd] x junction deviation        0.5000 mm (larger is faster)
[xsn] x switch min                3 [0=off,1=homing,2=limit,3=limit+homing]
[xsx] x switch max                2 [0=off,1=homing,2=limit,3=limit+homing]
[xsv] x search velocity        2000 mm/min
[xlv] x latch velocity         1000 mm/min
[xlb] x latch backoff            16.000 mm
[xzb] x zero backoff              2.000 mm
[yam] y axis mode                 1 [standard]
[yvm] y velocity maximum      10000 mm/min
[yfr] y feedrate maximum      10000 mm/min
[ytn] y travel minimum            0.000 mm
[ytm] y travel maximum          400.000 mm
[yjm] y jerk maximum         540000 mm/min^3 * 1 million
[yjh] y jerk homing            2000 mm/min^3 * 1 million
[yjd] y junction deviation        0.5000 mm (larger is faster)
[ysn] y switch min                3 [0=off,1=homing,2=limit,3=limit+homing]
[ysx] y switch max                2 [0=off,1=homing,2=limit,3=limit+homing]
[ysv] y search velocity        2000 mm/min
[ylv] y latch velocity         1000 mm/min
[ylb] y latch backoff            13.000 mm
[yzb] y zero backoff              2.000 mm
[zam] z axis mode                 1 [standard]
[zvm] z velocity maximum       5000 mm/min
[zfr] z feedrate maximum       2000 mm/min
[ztn] z travel minimum            0.000 mm
[ztm] z travel maximum           80.000 mm
[zjm] z jerk maximum           5400 mm/min^3 * 1 million
[zjh] z jerk homing             500 mm/min^3 * 1 million
[zjd] z junction deviation        0.5000 mm (larger is faster)
[zsn] z switch min                0 [0=off,1=homing,2=limit,3=limit+homing]
[zsx] z switch max                3 [0=off,1=homing,2=limit,3=limit+homing]
[zsv] z search velocity        1000 mm/min
[zlv] z latch velocity          100 mm/min
[zlb] z latch backoff             4.000 mm
[zzb] z zero backoff              2.000 mm
[aam] a axis mode                 1 [standard]
[avm] a velocity maximum      50000 deg/min
[afr] a feedrate maximum     200000 deg/min
[atn] a travel minimum           -1.000 deg
[atm] a travel maximum          600.000 deg
[ajm] a jerk maximum           5000 deg/min^3 * 1 million
[ajh] a jerk homing            5000 deg/min^3 * 1 million
[ajd] a junction deviation        0.0100 deg (larger is faster)
[ara] a radius value              0.1990 deg
[asn] a switch min                0 [0=off,1=homing,2=limit,3=limit+homing]
[asx] a switch max                0 [0=off,1=homing,2=limit,3=limit+homing]
[asv] a search velocity         600 deg/min
[alv] a latch velocity          100 deg/min
[alb] a latch backoff             5.000 deg
[azb] a zero backoff              2.000 deg
[bam] b axis mode                 0 [disabled]
[bvm] b velocity maximum       3600 deg/min
[bfr] b feedrate maximum       3600 deg/min
[btn] b travel minimum           -1.000 deg
[btm] b travel maximum           -1.000 deg
[bjm] b jerk maximum             20 deg/min^3 * 1 million
[bjd] b junction deviation        0.0500 deg (larger is faster)
[bra] b radius value              1.0000 deg
[cam] c axis mode                 0 [disabled]
[cvm] c velocity maximum       3600 deg/min
[cfr] c feedrate maximum       3600 deg/min
[ctn] c travel minimum           -1.000 deg
[ctm] c travel maximum           -1.000 deg
[cjm] c jerk maximum             20 deg/min^3 * 1 million
[cjd] c junction deviation        0.0500 deg (larger is faster)
[cra] c radius value              1.0000 deg
[p1frq] pwm frequency               100 Hz
[p1csl] pwm cw speed lo            1000 RPM
[p1csh] pwm cw speed hi            2000 RPM
[p1cpl] pwm cw phase lo           0.125 [0..1]
[p1cph] pwm cw phase hi           0.200 [0..1]
[p1wsl] pwm ccw speed lo           1000 RPM
[p1wsh] pwm ccw speed hi           2000 RPM
[p1wpl] pwm ccw phase lo          0.125 [0..1]
[p1wph] pwm ccw phase hi          0.200 [0..1]
[p1pof] pwm phase off             0.100 [0..1]
[g54x] g54 x offset               0.000 mm
[g54y] g54 y offset               0.000 mm
[g54z] g54 z offset               0.000 mm
[g54a] g54 a offset               0.000 deg
[g54b] g54 b offset               0.000 deg
[g54c] g54 c offset               0.000 deg
[g55x] g55 x offset              75.000 mm
[g55y] g55 y offset              75.000 mm
[g55z] g55 z offset               0.000 mm
[g55a] g55 a offset               0.000 deg
[g55b] g55 b offset               0.000 deg
[g55c] g55 c offset               0.000 deg
[g56x] g56 x offset               0.000 mm
[g56y] g56 y offset               0.000 mm
[g56z] g56 z offset               0.000 mm
[g56a] g56 a offset               0.000 deg
[g56b] g56 b offset               0.000 deg
[g56c] g56 c offset               0.000 deg
[g57x] g57 x offset               0.000 mm
[g57y] g57 y offset               0.000 mm
[g57z] g57 z offset               0.000 mm
[g57a] g57 a offset               0.000 deg
[g57b] g57 b offset               0.000 deg
[g57c] g57 c offset               0.000 deg
[g58x] g58 x offset               0.000 mm
[g58y] g58 y offset               0.000 mm
[g58z] g58 z offset               0.000 mm
[g58a] g58 a offset               0.000 deg
[g58b] g58 b offset               0.000 deg
[g58c] g58 c offset               0.000 deg
[g59x] g59 x offset               0.000 mm
[g59y] g59 y offset               0.000 mm
[g59z] g59 z offset               0.000 mm
[g59a] g59 a offset               0.000 deg
[g59b] g59 b offset               0.000 deg
[g59c] g59 c offset               0.000 deg
[g92x] g92 x offset               0.000 mm
[g92y] g92 y offset               0.000 mm
[g92z] g92 z offset               0.000 mm
[g92a] g92 a offset               0.000 deg
[g92b] g92 b offset               0.000 deg
[g92c] g92 c offset               0.000 deg
[g28x] g28 x position             0.000 mm
[g28y] g28 y position             0.000 mm
[g28z] g28 z position             0.000 mm
[g28a] g28 a position             0.000 deg
[g28b] g28 b position             0.000 deg
[g28c] g28 c position             0.000 deg
[g30x] g30 x position             0.000 mm
[g30y] g30 y position             0.000 mm
[g30z] g30 z position             0.000 mm
[g30a] g30 a position             0.000 deg
[g30b] g30 b position             0.000 deg
[g30c] g30 c position             0.000 deg

The problem appears to be that the cruise_velocity for a body only segment is being set to zero which in turn causes Nans to start propagating eventually leading to the hard alarm. This fix adds a check to prevent that from occurring.