Fix handling of context creation failure since r13978.

R=jkummerow@chromium.org
TEST=fast/js/concat-large-strings-crash.html

Review URL: https://codereview.chromium.org/12702008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13998 ce2b1a6d-e550-0410-aec6-3dcde31c8c00