Make sending only filled MsvAvFlags field for CHALLENGE message

[FeoOne]

I encountered a problem due to which MIC code verification on the server side in the gss-ntlmssp library consistently fails.
NTLM authentication used to implement DCE/RPC between Windows client and Linux server.

Environment
Client: Windows 10 22H2, standard MSRPC API.
Server: various Debian and RedHat based distributives, MIT Kerberos 5, gss-ntlmssp 1.2.

Experimentally, it was possible to establish that if there is no empty MsvAvFlags field (with a value of 0) in the TargetInfo list in the CHALLENGE message, the MIC hashes begins to match. The specification does not indicate that this field is required and there is an indirect indication that it may be missing in section 3.1.5.1.2:

If there is an AV_PAIR structure (section 2.2.2.1) with the AvId field set to MsvAvFlags, ... else ...

Apparently, Windows does not take this field into account when calculating MIC code that is transmitted in the AUTHENTICATE message.

[simo5]

Ohh this is a pretty good catch, I have been chasing this kind of issue for a while, but me and others could not figure out what was going on.

@FeoOne Could you please sign your commit so I can merge?

[FeoOne]

Yeah, this problem also gave us a lot of headaches. I hope this is a fix for the root cause, but so far tests are showing positive results.

Commit signed, thank you!