Fix valgrind error on I420ToNV12.  Do any conversions first, and work toward fastest, which produces more stable results for the tests that matter.
BUG=none
TEST=none
Review URL: https://webrtc-codereview.appspot.com/938005

git-svn-id: http://libyuv.googlecode.com/svn/trunk@445 16f28f9a-4ce2-e073-06de-1de4eb20be90