Fix SPNEGO one-hop interop against old IIS

IIS 6.0 and similar return a zero length reponse buffer in the last
SPNEGO packet when context initiation is performed without mutual
authentication.  In this case the underlying Kerberos mechanism has
already completed successfully on the first invocation, and SPNEGO
does not expect a mech response token in the answer.  If we get an
empty mech response token when the mech is complete during
negotiation, ignore it.

[ghudson@mit.edu: small code style and commit message changes]

ticket: 7797 (new)
target_version: 1.12.1
tags: pullup