avcodec/h264idct_template: Fix integer overflow in ff_h264_idct8_add()

Fixes: 2891/clusterfuzz-testcase-minimized-5881795457318912
Fixes: runtime error: signed integer overflow: 1551827968 - -775913984 cannot be represented in type 'int'

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