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.
R=@indutny @caitp
I'm going to send this upstream after make x64.release.quickcheck
completes...