Fixes: signed integer overflow: -1268324762623155200 * 8 cannot be represented in type 'long'
Fixes: 30123/clusterfuzz-testcase-minimized-ffmpeg_dem_THREEDOSTR_fuzzer-6710765123928064

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>