negotiate_kerberos_auth: Properly align NDR data (#2186)

negotiate_kerberos_auth: Properly align NDR data (#2186)

Resolves sporadic Negotiate/Kerberos auth failures that manifested
as proxy 407 loops or helper errors when decoding PAC data, depending
on ticket layout.

Previously, the parser advanced bpos by the remainder:
(bpos += bpos %n)
instead of padding to the next multiple of n.

For example, n = 4:
    bpos=5 (r=1): current: 6 (wrong), correct: 8
    bpos=6 (r=2): current: 8 (accidentally right)
    bpos=7 (r=3): current: 10 (wrong), correct: 8