Fix Visual Studio build warnings

Fixed a list of VS warnings. Warning message:
..\test\vp10_convolve_test.cc(34): warning C4244: 'initializing' : conversion
from 'ptrdiff_t' to 'int', possible loss of data

Change-Id: I9a1d3978a79fbb7b1ac028c5713ac72b6ff99172
1 file changed