Clear destinations to different values to detect returning with error code rather than doing the conversion.
BUG=296
TESTED=convert_unittest
R=johannkoenig@google.com

Review URL: https://webrtc-codereview.appspot.com/6349004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@923 16f28f9a-4ce2-e073-06de-1de4eb20be90