gssapi: Fix the return flags if we don't have delegation credentials.

The section 5.1 in RFC2744 for the ret_flags states:

GSS_C_DELEG_FLAG
  True - Delegated credentials are available via the
  delegated_cred_handle parameter.

So it should not be set if the delegated credentials are explicityl set
to no credentials.

Signed-off-by: Andreas Schneider <asn@samba.org>