Fixes: Assertion failure
Fixes: 23758/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5160954605338624.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 321ea59dac6538f92206bab0a2688fa24a25c4d2)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>