Fixes: division by zero
Fixes: 23162/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-4856420817436672

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 0e15b01b4e463d12128db2c15de7741637548347)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>