networkAgent: Only unregister if we've registered

It's possible to turn auto_register back on, but not immediately be
registered. Don't cause a (harmless) critical in this case.

https://bugzilla.gnome.org/show_bug.cgi?id=689884

js/ui/components/networkAgent.js

@@ -613,6 +613,7 @@ const NetworkAgent = new Lang.Class({
         this._vpnRequests = { };
 
         this._native.auto_register = false;
+        if (this._native.registered)
             this._native.unregister();
     },