tree-pretty-print: handle COMPONENT_REF with non-decl RHS

In the C++ front end, a COMPONENT_REF's second operand isn't always a
decl (at least at template parse time).  This patch makes the generic
pretty printer not ICE when printing such a COMPONENT_REF.

gcc/ChangeLog:

	* tree-pretty-print.cc (dump_generic_node) <case COMPONENT_REF>:
	Don't call component_ref_field_offset if the RHS isn't a decl.