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

@@ -613,7 +613,8 @@ const NetworkAgent = new Lang.Class({
         this._vpnRequests = { };
 
         this._native.auto_register = false;
-        this._native.unregister();
+        if (this._native.registered)
+            this._native.unregister();
     },
 
     _newRequest:  function(agent, requestId, connection, settingName, hints, flags) {