Affected the ffmpeg-filter_colorkey FATE-test (but only if the C version
of idct8_add is used and not e.g. the x86 SSE2 version).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

Fixes: left shift of negative value -107
Fixes: 20398/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_CAVS_fuzzer-5725389278412800
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>