Tiny Parser fix: init identifiers.

This bug went unnoticed because PreParserIdentifier and Handle<String> have
default ctors which create a null identifier, but this it not true for all
possible identifier types (especially pointers).

R=ulan@chromium.org
BUG=

Review URL: https://codereview.chromium.org/238253012

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20834 ce2b1a6d-e550-0410-aec6-3dcde31c8c00