network: Fix possible crash when changing airplane mode

When changing the airplane mode, either from the Bluetooth panel, or
through gnome-shell's status menu, we would receive signals from the
Rfkill gnome-settings-daemon service, even after the network panel was
closed, as we didn't unref' it. Except that the panel was mostly gone.
So splat.

https://bugzilla.gnome.org/show_bug.cgi?id=751482

panels/network/cc-network-panel.c

@@ -231,6 +231,7 @@ cc_network_panel_dispose (GObject *object)
                 g_cancellable_cancel (priv->cancellable);
 
         g_clear_object (&priv->cancellable);
+        g_clear_object (&priv->rfkill_proxy);
         g_clear_object (&priv->builder);
         g_clear_object (&priv->client);
         g_clear_object (&priv->modem_manager);