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/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/motionpixels.c

@@ -201,6 +201,8 @@ static int mp_get_vlc(MotionPixelsContext *mp, GetBitContext *gb)
     int i;
 
     i = (mp->codes_count == 1) ? 0 : get_vlc2(gb, mp->vlc.table, mp->max_codes_bits, 1);
+    if (i < 0)
+        return i;
     return mp->codes[i].delta;
 }