Fixes: division by 0
Fixes: 19123/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMAV2_fuzzer-5655493121146880

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit bf5c850b)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>