polkitAgent: Guard against repeated close() calls

We use the close() method to disconnect signal handlers set up in
init(), however the handler ID is only valid in the first call in
case the method is called more than once.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/221

js/ui/components/polkitAgent.js

@@ -201,7 +201,9 @@ var AuthenticationDialog = new Lang.Class({
     close(timestamp) {
         this.parent(timestamp);
 
-        Main.sessionMode.disconnect(this._sessionUpdatedId);
+        if (this._sessionUpdatedId)
+            Main.sessionMode.disconnect(this._sessionUpdatedId);
+        this._sessionUpdatedId = 0;
     },
 
     _ensureOpen() {