Fix tightvnc-filetransfer 64 bit buffer overflow

Seems there is a buffer overflow in x11vnc's tightvnc-filetransfer
code when compiling to 64 bits: a buffer is allocated using sizeof(int)
for the size, but then, data is copied into it using
sizeof(unsigned long).

Detected by GCC build with full warnings.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

libvncserver/tightvnc-filetransfer/filetransfermsg.c

@@ -393,7 +393,7 @@ FileTransferMsg
 CreateFileDownloadZeroSizeDataMsg(unsigned long mTime)
 {
 	FileTransferMsg fileDownloadZeroSizeDataMsg;
-	int length = sz_rfbFileDownloadDataMsg + sizeof(int);
+	int length = sz_rfbFileDownloadDataMsg + sizeof(unsigned long);
 	rfbFileDownloadDataMsg *pFDD = NULL;
 	char *pFollow = NULL;