Fixes: 15271/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5735262606327808
Fixes: signed integer overflow: -2147483648 - 8 cannot be represented in type 'int'

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