Fix endless loop when all vbuckets are available

Change-Id: Ib61a607b372ae2308305c9d22cdb0ff297d45562
Reviewed-on: http://review.couchbase.org/30378
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>