Fix assertion failure in mjsunit/regexp-stack-overflow.

TBR=machenbach@chromium.org

Review URL: https://codereview.chromium.org/364213003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22203 ce2b1a6d-e550-0410-aec6-3dcde31c8c00