Set client_socket to INVALID_SOCKET after closing.

src/sbbs3/answer.cpp

@@ -376,6 +376,7 @@ bool sbbs_t::answer()
 			ssh_mode = false;
 			pthread_mutex_unlock(&ssh_mutex);
 			close_socket(client_socket);
+			client_socket = INVALID_SOCKET;
 			useron.number = 0;
 			return false;
 		}