don't crash menelaus_auth on anonymous auth

Change-Id: I5f9dddad65381a92eab5341d509316a3dfa55273
Reviewed-on: http://review.couchbase.org/43151
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>