avcodec/svq3: Fix overflow in svq3_add_idct_c()

Fixes: runtime error: signed integer overflow: 2147392585 + 524288 cannot be represented in type 'int'
Fixes: 3348/clusterfuzz-testcase-minimized-4809500517203968

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