The demuxer code assumes the existence of a video stream

Fixes: assertion failure
Fixes: 21512/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5699660783288320

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