avcodec/simple_idct_template: Fix several integer overflows

Benchmarks with START_TIMER indicate that the code is faster with unsigned, (that is
with the patch), there was quite some fluctuation in the numbers so this may be just
random

Fixes: 811/clusterfuzz-testcase-6465493076541440

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