Fix embarrasing %s --> %d bug in PORT command

This bug was only triggered by the Netkit FTP client.  The built-in FTP
clients in Firefox and Chrome did not trigger it, neither did the
command line clients ncftp or lftp.  Hence, the PORT command does not
seem to very common (anymore).

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>