Robustification of nngnorb nnir message viewing

lisp/gnorb-gnus.el (gnorb-gnus-search-messages): We should only be
		   touching those variables at all under gnus 5.13

lisp/nngnorb.el: (nnir-run-gnorb): Various guards: check for a running
		 gnus, silently pass over malformed links, try
		 activating the link's group and drop it if we can't
		 activate, make sure the article number is a positive
		 integer, and return nil instead of an empty vector if
		 no messages are found.