GitLab

Fixes: out of array access
Fixes: 70741/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SNOW_fuzzer-5703668010647552

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg

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