Fix null ptr crash dumping TemplateTemplateParmDecl

The following program winds up with
D->getDefaultArgStorage().getInheritedFrom() == nullptr
during dumping the TemplateTemplateParmDecl corresponding to the
template parameter of i.

  template <typename>
  struct R;
  template <template <typename> class = R>
  void i();

This patch fixes the null pointer dereference.