avcodec/h264idct_template: fix multiple runtime error: signed integer overflow

Fixes: 857/clusterfuzz-testcase-5319093760557056

Benchmark changes from 335->333 (so if its not a random fluctuation then it would be faster)

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