Fixes: signed integer overflow: -635424002382840000 * 16 cannot be represented in type 'long'
Fixes: 33612/clusterfuzz-testcase-minimized-ffmpeg_dem_MV_fuzzer-5704741108711424

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Peter Ross <pross@xvid.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>