fix crash which could happen when closing an invalid socket

for some reason boost::asio doesn't account for this and lets us simply
die when/if this happens.

include/Client.h

@@ -127,6 +127,7 @@ private:
     std::string mDID;
     int mID = -1;
     std::chrono::time_point<TimeType> mLastPingTime;
+    std::mutex mDisconnectMtx;
 };
 
 // Returns a valid client, or nullptr if no such client exists