The decoder hardcodes that audio is stream_id = 1 so it does not
currently work with more or less than 1 video stream at st=0

Fixes: assertion failure
Fixes: 18602/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-6259277199310848

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