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

Fixes: 677/clusterfuzz-testcase-6635120628858880

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