cstp: AC_PKT_DISCONN payload length can be 0
cstp: AC_PKT_DISCONN payload length can be 0

Terminating ocserv while a connection is active can cause uninitialized
data to be printed to the terminal:

    Established DTLS connection (using GnuTLS). Ciphersuite (DTLS1.2)-(RSA)-(AES-128-GCM).
    Received server disconnect: 00 '<bunch of junk chars>'
    Send BYE packet: Server request
    Session terminated by server; exiting.

If we get an AC_PKT_DISCONN message with no payload, just skip printing
the disconnect reason.

Signed-off-by: Kevin Cernekee <3105dbb3dd63fbb4f6371cfb04adc6726f76c56b@gmail.com>

don-johnny/openconnect,mveplus/openconnect-client,nmav/openconnect-mine,mtmiller/openconnect,don-johnny/openconnect,matlockx/openconnect,matlockx/openconnect,atyndall/openconnect,brandt/openconnect,cernekee/openconnect,cernekee/openconnect,lins05/openconnect,atyndall/openconnect,liiir1985/openconnect,nmav/openconnect-mine,liiir1985/openconnect,mtmiller/openconnect,brandt/openconnect,brandt/openconnect,liiir1985/openconnect,atyndall/openconnect,cernekee/openconnect,mtmiller/openconnect,don-johnny/openconnect,cernekee/openconnect,mveplus/openconnect-client,don-johnny/openconnect,liiir1985/openconnect,mtmiller/openconnect,lins05/openconnect,matlockx/openconnect,atyndall/openconnect,brandt/openconnect,nmav/openconnect-mine,mveplus/openconnect-client,mveplus/openconnect-client,nmav/openconnect-mine,lins05/openconnect,matlockx/openconnect,lins05/openconnect