valgrind fix to CropNV12 unittest.  round crop amount to multiple of 2 so UV will be an even number.
BUG=350
TESTED=drmemory out\debug\libyuv_unittest.exe --gtest_catch_exceptions=0 --gtest_filter=*CropNV12
R=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/14159004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1050 16f28f9a-4ce2-e073-06de-1de4eb20be90