sanopt: Avoid crash on anonymous parameter [PR93436]

Here we crash when using -fsanitize=address -fdump-tree-sanopt because
the dumping code uses IDENTIFIER_POINTER on a null DECL_NAME.  Instead,
we can print "<anonymous>" in such a case.  Or we could avoid printing
that diagnostic altogether.

2020-01-26  Marek Polacek  <polacek@redhat.com>

	PR tree-optimization/93436
	* sanopt.c (sanitize_rewrite_addressable_params): Avoid crash on
	null DECL_NAME.