dcesrv_core: fix the auth3 for large ntlmssp messages

dcesrv_core: fix the auth3 for large ntlmssp messages

I know finding any real logic in reading the patch,
doesn't really show what's going on. I tried hard
to simplify it, but this is the only way I found
that fixed the test_auth_pad_ntlm_2889_auth3 test
without breaking other tests...

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14356

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
(cherry picked from commit 8b8e4ff1b19ba06821d774d0e1a8b1cad7f06120)