fix memory leaks in *ToI420 functions.

BUG=352
TESTED=drmemory out\debug\libyuv_unittest.exe --gtest_catch_exceptions=0 --gtest_filter=**ToI420_Opt
R=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/21289004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1060 16f28f9a-4ce2-e073-06de-1de4eb20be90