bpp-recv/defec.c: null-pointer deref in defecInit()

[pesco]

if defecInit is called after defecRecv processed a parameter packet
(serial=0) that specifies an invalid decoder algorithm, the parameters are
already in savedStatus so a subsequent defecInit will attempt to restore
the decoder but yield NULL, leading to a crash in

      currDecoder->init(currK, currN, maxLen);

this can happen if an invalid parameter packet is received before the system
enters deep sleep (-> defecInit after wakeup).

likewise, if the decoder parameters (k/n) are invalid, the unchecked call above will fail and subsequent decoder calls will access uninitialized memory or reuse stale values.