libvpx

fix a race condition caused by intra function pointer initialization

This patch fixed webm issue 962.
(https://code.google.com/p/webm/issues/detail?id=962)
The data races occurred when an encoder and a decoder were created
at the same time, and the function pointers were initialized twice.

Change-Id: I8851b753c4b4ad4767d6eea781b61f0ac9abb44b