gdm: reset cancellable if cancelled.

Otherwise the second attempt tot login after pressing "escape" key
on the login "freeze" and raise an exception about IOError operation
cancelled.

https://bugzilla.gnome.org/show_bug.cgi?id=681537

js/gdm/util.js

@@ -106,6 +106,7 @@ const ShellUserVerifier = new Lang.Class({
 
         if (this._userVerifier)
             this._userVerifier.call_cancel_sync(null);
+        this._cancellable = new Gio.Cancellable();
     },
 
     clear: function() {