Don't crash if the tm_zone field returned by localtime is NULL

BUG=chromium:158355
R=svenpanne@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/662093003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25129 ce2b1a6d-e550-0410-aec6-3dcde31c8c00