* lisp/mouse.el (context-menu-entry): Bind to make-sparse-keymap, not ignore.

Then where-is-internal does not run context-menu-map from the :filter,
and no context menu functions are called by e.g. describe-mode in wrong buffer
(bug#9923)

lisp/mouse.el

@@ -524,7 +524,7 @@ Some context functions add menu items below the separator."
   menu)
 
 (defvar context-menu-entry
-  `(menu-item ,(purecopy "Context Menu") ignore
+  `(menu-item ,(purecopy "Context Menu") ,(make-sparse-keymap)
               :filter (lambda (_) (context-menu-map)))
   "Menu item that creates the context menu and can be bound to a mouse key.")