Prevent NULL dereference in tablet_tool handler

In case a tool was removed, but not yet destroyed by the client, the
tool_client's tool can be NULL. We have to check that as well in the
set_cursor handler to prevent using inert resources

types/tablet_v2/wlr_tablet_v2_tool.c

@@ -23,7 +23,7 @@ static void handle_tablet_tool_v2_set_cursor(struct wl_client *client,
 		struct wl_resource *surface_resource,
 		int32_t hotspot_x, int32_t hotspot_y) {
 	struct wlr_tablet_tool_client_v2 *tool = tablet_tool_client_from_resource(resource);
-	if (!tool) {
+	if (!tool || !tool->tool) {
 		return;
 	}