libvncserver: mitigate FORTIFY crash in listenerRun()

With the previous commit acac779
the select() call now times out, opening up for a race condition
where rfbShutdownSockets() would set the listening sockets to -1,
the select would time out and the FD_ISSET() call encounter a -1
socket.

This is not 100% threadsafe but better than before.

src/libvncserver/main.c

@@ -668,9 +668,9 @@ listenerRun(void *data)
 
 	/* If there is something on the listening sockets, handle new connections */
 	len = sizeof (peer);
-	if (FD_ISSET(screen->listenSock, &listen_fds)) 
+	if (screen->listenSock != RFB_INVALID_SOCKET && FD_ISSET(screen->listenSock, &listen_fds))
 	    client_fd = accept(screen->listenSock, (struct sockaddr*)&peer, &len);
-	else if (FD_ISSET(screen->listen6Sock, &listen_fds))
+	else if (screen->listen6Sock != RFB_INVALID_SOCKET && FD_ISSET(screen->listen6Sock, &listen_fds))
 	    client_fd = accept(screen->listen6Sock, (struct sockaddr*)&peer, &len);
 
 	if(client_fd >= 0)