print: disconnect source_changed_handler on printeroptionwidget finalize

Printing a web page without window.print (which still segfault),
that is Ctrl+P is fixed (does not attempt to execute the
source_changed_handler on a printeroptionwidget that has been
destroyed) by disconnecting this handler in the printeroptionwidget
finalize.

https://bugzilla.gnome.org/show_bug.cgi?id=696622

gtk/gtkprinteroptionwidget.c

@@ -152,6 +152,8 @@ gtk_printer_option_widget_finalize (GObject *object)
   
   if (priv->source)
     {
+      g_signal_handler_disconnect (priv->source,
+				   priv->source_changed_handler);
       g_object_unref (priv->source);
       priv->source = NULL;
     }