Fixes: 50993/clusterfuzz-testcase-minimized-ffmpeg_dem_JACOSUB_fuzzer-6722544461283328
Fixes: signed integer overflow: 48214448 * 60 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>