Merged r13562 into trunk branch.

Work around a bug in Clang that optimizes away a NULL check

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/12089074

git-svn-id: https://v8.googlecode.com/svn/trunk@13563 ce2b1a6d-e550-0410-aec6-3dcde31c8c00