Make sure to only update the proxy array if it's still there

The proxy array of the browser is unset during disposal, so we
may end up trying to work with a NULL value.

midori/midori-browser.c

@@ -4009,6 +4009,7 @@ gtk_notebook_switch_page_cb (GtkWidget*       notebook,
     gtk_window_set_title (GTK_WINDOW (browser), window_title);
     g_free (window_title);
 
+    if (browser->proxy_array)
         katze_item_set_meta_integer (KATZE_ITEM (browser->proxy_array), "current",
                                      midori_browser_get_current_page (browser));
     g_object_notify (G_OBJECT (browser), "uri");