Fixes: 15246/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MOTIONPIXELS_fuzzer-5168534407086080
Fixes: runtime error: index -1 out of bounds for type 'HuffCode [16]'

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>