avcodec/h264idct_template: Fix integer overflow in ff_h264_idct_add()

Fixes: runtime error: signed integer overflow: 26215360 + 2121330944 cannot be represented in type 'int'
Fixes: 2809/clusterfuzz-testcase-minimized-4785181833560064

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
