avcodec/h264idct_template: Fix integer overflow in ff_h264_idct8_add

Fixes: signed integer overflow: 452986184 - -2113885312 cannot be represented in type 'int'
Fixes: 4196/clusterfuzz-testcase-minimized-5580648594014208

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