Only show "Back" when the marker stack is non-empty

* lisp/menu-bar.el (menu-bar-goto-menu): Use it.

* lisp/progmodes/xref.el (xref-marker-stack-empty-p): New function.

lisp/ChangeLog

+2015-01-03  Dmitry Gutov  <dgutov@yandex.ru>
+
+	* progmodes/xref.el (xref-marker-stack-empty-p): New function.
+
+	* menu-bar.el (menu-bar-goto-menu): Use it.
+
 2015-01-03  Dmitry Gutov  <dgutov@yandex.ru>
 
 	* progmodes/xref.el (xref--window-configuration): New variable.

lisp/menu-bar.el

@@ -381,6 +381,7 @@
 
     (bindings--define-key menu [xref-pop]
       '(menu-item "Back" xref-pop-marker-stack
+                  :visible (not (xref-marker-stack-empty-p))
                   :help "Back to the position of the last search"))
 
     (bindings--define-key menu [xref-apropos]

lisp/progmodes/xref.el

@@ -301,6 +301,10 @@ backward."
       (let ((marker (ring-remove ring)))
         (set-marker marker nil nil)))))
 
+(defun xref-marker-stack-empty-p ()
+  "Return t if the marker stack is empty; nil otherwise."
+  (ring-empty-p xref--marker-ring))
+
 
 (defun xref--goto-location (location)
   "Set buffer and point according to xref-location LOCATION."