Fixes: Timeout (195sec -> 2ms)
Fixes: 16735/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5090676403863552

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>