Merged r15167 into trunk branch.

Add missing NULL check to TypeFeedbackOracle::CompareTypes()

R=verwaest@chromium.org
BUG=

Review URL: https://codereview.chromium.org/17030011

git-svn-id: https://v8.googlecode.com/svn/trunk@15168 ce2b1a6d-e550-0410-aec6-3dcde31c8c00