org-irc: Fix port handling

* lisp/org-irc.el (org-irc-visit-erc): Fix "stringp, nil".

Reported-by: "numbchild@gmail.com" <numbchild@gmail.com>
<http://lists.gnu.org/archive/html/emacs-orgmode/2017-06/msg00541.html>

lisp/org-irc.el

@@ -205,7 +205,8 @@ default."
   (require 'erc)
   (require 'erc-log)
   (let* ((server (car (car link)))
-	 (port (or (string-to-number (cadr (pop link))) erc-default-port))
+	 (port (let ((p (cadr (pop link))))
+		 (if p (string-to-number p) erc-default-port)))
 	 (server-buffer)
 	 (buffer-list
 	  (erc-buffer-filter