Reset auth_data when called for an invalid connection

Change-Id: Icd9d8b14e51e12348f2702f1ea60bf543a6255d6
Reviewed-on: http://review.couchbase.org/38690
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>