authPrompt: disassociate from userVerifier when destroyed

Otherwise, it won't get GC'd and we'll end up potentially calling
its signal handlers after destruction.

https://bugzilla.gnome.org/show_bug.cgi?id=683437

js/gdm/authPrompt.js

@@ -123,6 +123,8 @@ const AuthPrompt = new Lang.Class({
 
     _onDestroy: function() {
         this._userVerifier.clear();
+        this._userVerifier.disconnectAll();
+        this._userVerifier = null;
     },
 
     _initButtons: function() {