Fix use-of-uninitialized-value introduced in r24700

LOG=N
BUG=chromium:425146
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/665243002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24760 ce2b1a6d-e550-0410-aec6-3dcde31c8c00