Fixes: null pointer dereference
Fixes: 20768/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5638648978735104.fuzz

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>