When the socket has been closed already, cl->sock is -1.  Don't pass
that to FD_SET() etc.

diff -up a/LibVNCServer-0.8.2/libvncserver/main.c b/LibVNCServer-0.8.2/libvncserver/main.c
--- a/LibVNCServer-0.8.2/libvncserver/main.c	2007-07-24 20:33:55.000000000 +0200
+++ b/LibVNCServer-0.8.2/libvncserver/main.c	2007-07-25 15:48:58.000000000 +0200
@@ -497,9 +497,12 @@ clientInput(void *data)
 	int n, sock;
 
 	LOCK(cl->updateMutex);
-	sock = cl->sock;
+	sock = dup(cl->sock);
 	UNLOCK(cl->updateMutex);
 
+	if (sock == -1)
+	    break;
+
 	FD_ZERO(&rfds);
 	FD_SET(sock, &rfds);
 	FD_ZERO(&efds);
@@ -515,11 +518,13 @@ clientInput(void *data)
 	n = select(sock + 1, &rfds, &wfds, &efds, &tv);
 	if (n < 0) {
 	    rfbLogPerror("ReadExact: select");
+	    close(sock);
 	    break;
 	}
 	if (n == 0) /* timeout */
 	{
             rfbSendFileTransferChunk(cl);
+	    close(sock);
 	    continue;
         }
         
@@ -530,6 +535,8 @@ clientInput(void *data)
         if (FD_ISSET(sock, &rfds) || FD_ISSET(sock, &efds))
             rfbProcessClientMessage(cl);
 
+	close(sock);
+
 	LOCK(cl->updateMutex);
 	if (cl->sock == -1) {
 	    UNLOCK(cl->updateMutex);