Fixes: null pointer dereference
Fixes: 17828/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5645915116797952
Fixes: Ticket8147

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