Apply race condition fix to mux (#78) (#88)

Apply fix described in #56 to mux_node

Signed-off-by: Andrew Best <andrew.best@tri.global>
(cherry picked from commit 7dc8c43)

Co-authored-by: andrewbest-tri <43861892+andrewbest-tri@users.noreply.github.com>

topic_tools/src/mux_node.cpp

@@ -64,6 +64,10 @@ void MuxNode::make_subscribe_unsubscribe_decisions()
 
 void MuxNode::process_message(std::shared_ptr<rclcpp::SerializedMessage> msg)
 {
+  std::scoped_lock lock(pub_mutex_);
+  if (!pub_) {
+    return;
+  }
   pub_->publish(*msg);
 }