DBusTransport: don't copy DBusAuth's GUID to expected_guid

DBusTransport: don't copy DBusAuth's GUID to expected_guid

There was previously a leak in _dbus_transport_get_is_authenticated, if
we hit OOM when copying the server's actual GUID. This would fail the
"SHA1 connection test" if _dbus_iterate was modified to allocate and free
one extra pointer per iteration.

Rather than adding OOM handling everywhere, if we avoid strdup'ing the
GUID, and just re-fetch the const string from the DBusAuth object on demand
instead, we go back to a situation where _dbus_transport_get_is_authenticated
can't fail.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=33126