w: Unlike err(3), xo_err(3) won't accept a null format string.

MFC after:	1 week
X-MFC-with:	d90ff31a
Sponsored by:	Klara, Inc.
Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D38192

usr.bin/w/w.c

@@ -400,7 +400,7 @@ main(int argc, char *argv[])
 		ep->args = fmt_argv(kvm_getargv(kd, ep->kp, argwidth),
 		    ep->kp->ki_comm, NULL, MAXCOMLEN);
 		if (ep->args == NULL)
-			xo_err(1, NULL);
+			xo_err(1, "fmt_argv");
 	}
 	/* sort by idle time */
 	if (sortidle && ehead != NULL) {