Fix the 16-bit copy in convolve test to avoid the use of
uninitialized values. This fixes unit tests
C_8/ConvolveTest.Avg/*

bug=aom:19

The issue was discovered by a proper fix to CONVERT_TO_BYTEPTR()
in f16387bc.

Change-Id: Ibf0acdf4fce2ca2cd5ab5e581d72eca173d8500d