avcodec/dcadsp: Fix 2 runtime error: signed integer overflow: -1958094138 - 1078906344 cannot be represented in type 'int'

Fixes: 722/clusterfuzz-testcase-5711268868521984

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