Fix GtkRecentChooserDialog segfault

C-function *gtk_recent_chooser_dialog_new* is similar to *gtk_dialog_new_with_buttons*.

From GtkDialog documentation:
> After flags , button text/response ID pairs should be listed, with a NULL pointer ending the list.

Currently gtk_recent_chooser_dialog_new is called with two *c_strs* at the end without the NULL guard. This causes a segfault in one of the rgtk examples.