Fixes: signed integer overflow: -2515507630940093440 * 4 cannot be represented in type 'long'
Fixes: 46318/clusterfuzz-testcase-minimized-ffmpeg_dem_GENH_fuzzer-5009637474172928

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>