swarm_manager: avoid deadlock in shutdown

shutdown can cause the needSocketCb_ to pass a null socket. In this
case we can directly return instead of locking

GitLab: #995
Change-Id: I8ad744e494ea0bd3fe15f221a09feeb44e9aec83

src/jamidht/swarm/swarm_manager.cpp

@@ -339,7 +339,7 @@ SwarmManager::tryConnect(const NodeId& nodeId)
         needSocketCb_(nodeId.toString(),
                       [w = weak(), nodeId](const std::shared_ptr<dhtnet::ChannelSocketInterface>& socket) {
                           auto shared = w.lock();
-                          if (!shared)
+                          if (!shared || shared->isShutdown_)
                               return true;
                           if (socket) {
                               shared->addChannel(socket);