Fix overflow issue in SSSE3 32x32 quantization

The 32x32 quantization process can potentially have the intermediate
stacks over 16-bit range, thereby causing enc/dec mismatch. This commit
fixes this overflow issue in the SSSE3 implementation, as well as the
prototype, of 32x32 quantization.

This fixes issue 607 from webm@googlecode.

Change-Id: I85635e6ca236b90c3dcfc40d449215c7b9caa806