avcodec/vp8dsp: Fixes: runtime error: signed integer overflow: 1330143360 - -1023040530 cannot be represented in type 'int'

Fixes: 1406/clusterfuzz-testcase-minimized-5064865125236736

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