avcodec/h264idct_template: Fix integer overflows in ff_h264_idct8_add()

Fixes: runtime error: signed integer overflow: -503316480 + -2013265038 cannot be represented in type 'int'
Fixes: 3805/clusterfuzz-testcase-minimized-6578427831255040

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