issue-919 Fixed "memory leak" in action clients (#920)

Whenever a call is made to `rclcpp_action::Client::wait_for_action_server`
a weak pointer to an Event object gets added to the graph_event_ vector
of the NodeGraph interface. This vector will be cleared on a node graph
change event, but if no such event occurs the weak pointer will be stuck
in the vector.  Furthermore, if client code issues repeated calls to
`wait_for_action_server` the vector will keep growing.

The fix moves the Event object creation right after the early return from
`wait_for_action_server` so that the Event object is not created in the
case that the server is known to be present and therefore there is no
need to wait for a node graph change event to occur.

Signed-off-by: Adrian Stere <astere@clearpath.ai>
Signed-off-by: Ivan Santiago Paunovic <ivanpauno@ekumenlabs.com>

rclcpp_action/src/client.cpp

@@ -165,11 +165,11 @@ ClientBase::wait_for_action_server_nanoseconds(std::chrono::nanoseconds timeout)
   if (!node_ptr) {
     throw rclcpp::exceptions::InvalidNodeError();
   }
-  auto event = node_ptr->get_graph_event();
   // check to see if the server is ready immediately
   if (this->action_server_is_ready()) {
     return true;
   }
+  auto event = node_ptr->get_graph_event();
   if (timeout == std::chrono::nanoseconds(0)) {
     // check was non-blocking, return immediately
     return false;