libvpx

fixed the reading too many bytes

In subpel_avg_variance functions, code similar to the following

punpkldq m2, [addr]

actually reads 8 bytes. For functions that are supposed to work on
buffers only have less 8 bytes a line, this caused valgrind error
of reading uninitialized memory.

Change-Id: I2a4c079dbdbc747829bd9e2ed85f0018ad2a3a34