Fixes regression since 9ad47762
Fixes: out of array access
Fixes: 22172/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5658535590625280

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>