interpolate stores/restores an extra byte, so asan complains about the potential danger.  This pads the buffer by 16, which is what the user is expected to do, for now at least.
BUG=81
TESTED=asan on unittests
Review URL: https://webrtc-codereview.appspot.com/799005

git-svn-id: http://libyuv.googlecode.com/svn/trunk@349 16f28f9a-4ce2-e073-06de-1de4eb20be90