Specifying a defined Graph argument to RT::Graph::Tickets->TicketLinks
is only used internally when it is called recursively.  Since Graph is
expected to be an existing GraphViz object if defined, it never makes
sense to start with anything but an undefined Graph parameter.

This prevents a user-supplied Graph parameter from having ->add_node
called on it.  Since the Graph parameter could contain a Perl package
name, it previously provided a means to call to ->add_node on arbitrary
Perl packages already loaded into memory.  While of unlikely utility,
there's no reason to allow such behaviour.

Fixes part of CVE-2011-4458.