fix wait for service memory leak bug (#656)

* when call wait for service in an while loop, the event will be make forever and never release
* fix it by: creating it when we need

Signed-off-by: reed-lau <geoliuwei@gmail.com>

rclcpp/src/rclcpp/client.cpp

@@ -118,7 +118,6 @@ ClientBase::wait_for_service_nanoseconds(std::chrono::nanoseconds timeout)
   if (!node_ptr) {
     throw InvalidNodeError();
   }
-  auto event = node_ptr->get_graph_event();
   // check to see if the server is ready immediately
   if (this->service_is_ready()) {
     return true;
@@ -127,6 +126,7 @@ ClientBase::wait_for_service_nanoseconds(std::chrono::nanoseconds timeout)
     // check was non-blocking, return immediately
     return false;
   }
+  auto event = node_ptr->get_graph_event();
   // update the time even on the first loop to account for time spent in the first call
   // to this->server_is_ready()
   std::chrono::nanoseconds time_to_wait =