Fixes: assertion failure
Fixes: 23434/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5227750851084288.fuzz

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 c517c3f4741b6897ea952d1fba199c93c5217cfe)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>