avcodec/h264addpx_template: Fixes integer overflows

Fixes: signed integer overflow: 512 + 2147483491 cannot be represented in type 'int'
Fixes: 4780/clusterfuzz-testcase-minimized-4709066174627840

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