Fixes infinite loop
Fixes: 6858/clusterfuzz-testcase-ffmpeg_AV_CODEC_ID_MSMPEG4V3_fuzzer-4681563766784000
Fixes: 6890/clusterfuzz-testcase-ffmpeg_AV_CODEC_ID_WMV1_fuzzer-4756103142309888

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