Fixes: signed integer overflow: 6000 * -2147483648 cannot be represented in type 'int'
Fixes: 25700/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-6578316302352384

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