Fix debug message

This outputs the config not the number of read bytes.
The recv_queue_.back() call was also missing lock protection.

src/wesnothd_connection.cpp

@@ -319,12 +319,13 @@ void wesnothd_connection::handle_read(const boost::system::error_code& ec, std::
 	std::istream is(&read_buf_);
 	config data;
 	read_gz(data, is);
+	if(!data.empty()) DBG_NW << "Received:\n" << data;
+
 	{
 		std::lock_guard<std::mutex> lock(recv_queue_mutex_);
 		recv_queue_.emplace_back(std::move(data));
 	}
 
-	DBG_NW << "Received " << recv_queue_.back() << " bytes.\n";
 	recv();
 }