Fixes: out of array access
Fixes: 62678/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-4858264984354816

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Kieran Kunhya <kierank@obe.tv>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>