Suppress libyuv uninitialized read in CopyRow_AVX

BUG=libyuv:377
R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/25919004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7494 4adac7df-926f-26a2-2b94-8c16560cd09d