No longer OOM on invalid string length.

R=ishell@chromium.org
BUG=v8:3060
LOG=Y

Review URL: https://codereview.chromium.org/207613005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20202 ce2b1a6d-e550-0410-aec6-3dcde31c8c00