StaticSingleThreadedExecutor throws rcl error (couldn't add guard condition to wait set) if node goes out of scope before spin

[brawner]

When expanding the test_executor unit test to StaticSingleThreadedExecutor (in #1222), the temporary node test fails for this executor.

terminate called after throwing an instance of 'std::runtime_error'
  what():  Executor waitable: couldn't add guard condition to wait set
Aborted (core dumped)

Example of temporary node unit test.

rclcpp/rclcpp/test/rclcpp/test_executor.cpp

Lines 65 to 69 in 0986fbb

	TEST_F(TestExecutors, addTemporaryNode) {
	rclcpp::executors::SingleThreadedExecutor executor;
	executor.add_node(std::make_shared<rclcpp::Node>("temporary_node"));
	EXPECT_NO_THROW(executor.spin_some());
	}