alternative-status-menu: fix criticals when opening the menu

After enabling, the user menu was keeping a reference to a
destroyed actor, and then was updating it from signals. Updating
this (a StEntry) caused g_criticals because of a NULL ClutterText.
Instead we can null out the actor entirely, which will stop
the signal handler from doing any harm.

extensions/alternative-status-menu/extension.js

@@ -95,6 +95,9 @@ function createSubMenu() {
 	this._session.ShutdownRemote();
     }));
     this.menu.addMenuItem(item);
+
+    // clear out this to avoid criticals
+    this._suspendOrPowerOffItem = null;
 }
 
 // Put your extension initialization code here