avcodec/dss_sp: Fix multiple runtime error: signed integer overflow: -15699 * -164039 cannot be represented in type 'int'

Fixed: 1409/clusterfuzz-testcase-minimized-5237365020819456

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
