heimdal/gssapi: Allow a NULL authenticator

heimdal/gssapi: Allow a NULL authenticator

Some non-GSSAPI implementations that instead try to create compatible packets by wrapping krb5_mk_req()
can trigger a NULL authenticator here.  Assume this to be equvilent to specifying an all-zero
channel bindings and some reasonable (fixed) flags.

This was seen in the wild, with a Huawei Unified Storage System S5500 V3 against the AD DC

Original patch by Andrew Bartlett, restructured by Douglas Bagnall

Cherry-picked from upstream GIT 0a5de96d72cdea9e465412d7dba1e5d13e53dc09
which is the merge of https://github.com/heimdal/heimdal/pull/134

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11425
Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Stefan Metzmacher <metze@samba.org>