avcodec/vp8dsp: vp7_luma_dc_wht_c: Fix multiple runtime error: signed integer overflow: -1366381240 + -1262413604 cannot be represented in type 'int'

Fixes: 1440/clusterfuzz-testcase-minimized-5785716111966208

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