Fix for insufficient NTLM buffer length on SSPI

The default NTLM buffer length, as defined in tds/sspi.c (NTLMBUF_LEN =
4096) is insufficient in some environments.

Make InitializeSecurityContext call allocate buffer dynamically as needed.

Signed-off-by: Samuel Ferencik <samuel.ferencik@barclays.com>
Signed-off-by: Frediano Ziglio <freddy77@gmail.com>