(rmail-output-menu): Don't call expand-file-name.

Do nothing if rmail-secondary-file-menu return nil.

lisp/mail/rmailout.el

@@ -38,11 +38,13 @@ or more generally it may be any kind of expression that returns
 a file name as a string.")
 
 (defun rmail-output-menu (event)
-  "Output current message to another Rmail file, chosen with a menu."
+  "Output current message to another Rmail file, chosen with a menu.
+The variables `rmail-secondary-file-directory' and
+`rmail-secondary-file-regexp' control which files are offered in the menu."
   (interactive "e")
-  (rmail-output-to-rmail-file
-   (expand-file-name (rmail-secondary-file-menu event)
-		     rmail-secondary-file-directory)))
+  (let ((file-name (rmail-secondary-file-menu event)))
+    (if file-name
+	(rmail-output-to-rmail-file file-name))))
 
 ;;; There are functions elsewhere in Emacs that use this function; check
 ;;; them out before you change the calling method.