One of the source lines for ScaleRowDown4Int_SSE2 was
incorrect leading to a bad read.  Add a test to catch
this in the future.
Review URL: http://webrtc-codereview.appspot.com/242006

git-svn-id: http://libyuv.googlecode.com/svn/trunk@39 16f28f9a-4ce2-e073-06de-1de4eb20be90