v8: fix template literal NULL pointer deref

Fixes a NULL pointer dereference with unterminated template literals.

This is a back-port of commit v8/v8-git-mirror@02218ad from the V8
master branch, see https://code.google.com/p/v8/issues/detail?id=3820.

PR-URL: https://github.com/iojs/io.js/pull/534


Reviewed-By: Caitlin Potter <caitpotter88@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>