ntlm: Clarify details on NTLM phase 3 decoding

ntlm: Clarify details on NTLM phase 3 decoding

The code was not very clear if we accept the base64 decode if the
NTLM challenge was truncated or not.  Move the related code lines
closer to where buf is first used and comment that we are not concerned
about any truncation.

If the decoded result is truncated, the NTLM server side will reject
our new response to the challenge as it will be incorrect.  The
buffer size is fixed and known to be in a cleared state before the
decode starts.

Resolves: TOB-OVPN-14
Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230802113149.36497-1-dazo+openvpn@eurephia.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26919.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit f19391139836aa07312cf5b3ebbd00941d22ddc7)