Fixes: signed integer overflow: -1300248894420254720 * 16 cannot be represented in type 'long'
Fixes: 30879/clusterfuzz-testcase-minimized-ffmpeg_dem_DCSTR_fuzzer-5094464215449600

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>