Avoid more errors by not trying to re-create a stack win with height == 0

src/layout.c

@@ -460,6 +460,7 @@ void render_container(xcb_connection_t *conn, Container *container) {
                 }
 
                 /* Prepare the pixmap for usage */
+                if (num_clients != 1)
                         cached_pixmap_prepare(conn, &(stack_win->pixmap));
 
                 int current_row = 0, current_col = 0;