Fixes: Out of array access
Fixes: 37030/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5387719147651072
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>