singleselection: check model range before getting item

It is invalid to request an item from a model outside the range of the
selection. In some cases, we can avoid the check since we know there was
an item added to the model.

gtk/gtksingleselection.c

@@ -182,7 +182,9 @@ gtk_single_selection_items_changed_cb (GListModel         *model,
     {
       if (self->autoselect)
         {
+          if (added > 0 || g_list_model_get_n_items (self->model) > 0)
             self->selected_item = g_list_model_get_item (self->model, 0);
+
           if (self->selected_item)
             {
               self->selected = 0;