TelnetServer: Protect the Client object during drain_socket()

If parsing a command causes us to disconnect the client, we have to
defer deletion of the Client object until we're done with the socket.

Servers/TelnetServer/Client.cpp

@@ -57,6 +57,7 @@ Client::Client(int id, RefPtr<Core::TCPSocket> socket, int ptm_fd)
 
 void Client::drain_socket()
 {
+    NonnullRefPtr<Client> protect(*this);
     while (m_socket->can_read()) {
         auto buf = m_socket->read(1024);