In Q420ToI420 the variable halfheight is initialized but not used.  Change it to instantiate the variable but do not initialize it.  It will be assigned conditionally later.  This warning raised in xcode.
BUG=353
TESTED=local build still works
R=harryjin@google.com, noahric@chromium.org

Review URL: https://webrtc-codereview.appspot.com/13299004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1053 16f28f9a-4ce2-e073-06de-1de4eb20be90