avcodec/motionpixels: Check for vlc error in mp_get_vlc()
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/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit
930cdef80ab695132d3de2128c3c23f2d698918b)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>