Fixes: division by zero
Fixes: 20436/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5763229752229888
Fixes: 20503/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-4841641154445312

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>