Revert "Remove unnecessary check in RegExpExecStub."

This reverts r22203 and r22205.

TBR=danno@chromium.org

Review URL: https://codereview.chromium.org/369063005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22211 ce2b1a6d-e550-0410-aec6-3dcde31c8c00