Add mutex for connections list in destructor

Signed-off-by: Jose Luis Rivero <jrivero@osrfoundation.org>

gazebo/common/Event.hh

@@ -611,6 +611,7 @@ namespace gazebo
     template<typename T>
     EventT<T>::~EventT()
     {
+      std::lock_guard<std::mutex> lock(this->mutex);
       this->connections.clear();
     }