MB-12042 check for existence of vbucket in warmup callback

Change-Id: Iab0ad237392d681f28b91c29dc51b7631141e187
Reviewed-on: http://review.couchbase.org/40849
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>