Null-out the padeditor's reference to ace once it has been distroyed.

This enables the guards placed in `padeditor.disable` to work. On `window.unload` the pad's connection would get closed which would lead to the editor becomming disabled. If the editor had already been `disposed` then the call to `disable` would raise the exception:

  `Uncaught TypeError: Cannot call method 'ace_setProperty' of null`

static/js/pad_editor.js

@@ -141,6 +141,7 @@ var padeditor = (function()
       if (self.ace)
       {
         self.ace.destroy();
+        self.ace = null;
       }
     },
     disable: function()