heap-buffer-overflow read in nsvg__parseColorRGB()

[invd]

See #178 for context and program setup. Reproducible on newest master commit 9da543e.

==21508==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000000020 [...]
READ of size 1 at 0x602000000020 thread T0
    #0 0x563f49e09bc1 in nsvg__parseColorRGB /nanosvg/src/nanosvg.h:1268:25
    #1 0x563f49e06e63 in nsvg__parseColor /nanosvg/src/nanosvg.h:1467:10
    #2 0x563f49e03609 in nsvg__parseAttr /nanosvg/src/nanosvg.h:1800:22
    #3 0x563f49df7d78 in nsvg__parseAttribs /nanosvg/src/nanosvg.h:1913:4
    #4 0x563f49de632b in nsvg__startElement /nanosvg/src/nanosvg.h:2769:3
    #5 0x563f49de4422 in nsvg__parseElement /nanosvg/src/nanosvg.h:324:3
    #6 0x563f49de2549 in nsvg__parseXML /nanosvg/src/nanosvg.h:348:4
    #7 0x563f49de45f8 in nsvgParse /nanosvg/src/nanosvg.h:3005:2
    #8 0x563f49deeb63 in nsvgParseFromFile /nanosvg/src/nanosvg.h:3038:10

Reproducer file input:

CC @memononen @oehhar .