avcodec/dss_sp: Fix runtime error: signed integer overflow: 2147481189 + 4096 cannot be represented in type 'int'

Fixes: 1441/clusterfuzz-testcase-minimized-6223152357048320

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