Block attempts to connect to the client (#10589)

A Minetest peer initiates a connection by sending a packet with an invalid peer_id, for whatever reason the code for doing this ran on both the client and the server meaning you could connect to a client if you knew what the address:port tuple it was listening on.

src/network/connection.h

@@ -809,6 +809,11 @@ protected:
 	void putEvent(ConnectionEvent &e);
 
 	void TriggerSend();
+	
+	bool ConnectedToServer() 
+	{
+		return getPeerNoEx(PEER_ID_SERVER) != nullptr;
+	}
 private:
 	MutexedQueue<ConnectionEvent> m_event_queue;