loginDialog: don't try to stop work spinner if it's already gone

This can happen if the dialog gets reset at the wrong moment.

https://bugzilla.gnome.org/show_bug.cgi?id=693757

js/gdm/loginDialog.js

@@ -1063,6 +1063,7 @@ const LoginDialog = new Lang.Class({
                                transition: 'linear',
                                onCompleteScope: this,
                                onComplete: function() {
+                                   if (this._workSpinner)
                                        this._workSpinner.stop();
                                }
                              });