When using C version of I420Interpolate for msan, a 50% interpolation
would cause stride to be cast to int, which could cause erroneous
memory reads on 64 bit build.
This CL makes the stride use ptrdiff_t for HalfRow_C

BUG=libyuv:582
TESTED=try bots tests
R=dhrosa@google.com

Review URL: https://codereview.chromium.org/1872953002 .