Fixes: signed integer overflow: 65535 * 65312 cannot be represented in type 'int'
Fixes: 16704/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-6294115603447808

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