Avoid unecessary creation of MultiThreadedExecutor (#3090) (#3096)

(cherry picked from commit 8cd4d47)

Signed-off-by: solonovamax <solonovamax@12oclockpoint.com>
Co-authored-by: solo <solonovamax@12oclockpoint.com>

Author: mergify[bot]

rclcpp_components/src/component_container_mt.cpp

@@ -23,16 +23,16 @@ int main(int argc, char * argv[])
   /// Component container with a multi-threaded executor.
   rclcpp::init(argc, argv);
 
-  auto exec = std::make_shared<rclcpp::executors::MultiThreadedExecutor>();
-  auto node = std::make_shared<rclcpp_components::ComponentManager>();
+  rclcpp::executors::MultiThreadedExecutor::SharedPtr exec = nullptr;
+  const auto node = std::make_shared<rclcpp_components::ComponentManager>();
   if (node->has_parameter("thread_num")) {
     const auto thread_num = node->get_parameter("thread_num").as_int();
     exec = std::make_shared<rclcpp::executors::MultiThreadedExecutor>(
       rclcpp::ExecutorOptions{}, thread_num);
-    node->set_executor(exec);
   } else {
-    node->set_executor(exec);
+    exec = std::make_shared<rclcpp::executors::MultiThreadedExecutor>();
   }
+  node->set_executor(exec);
   exec->add_node(node);
   exec->spin();
 }