MB-11172: Check vbucket existence before deregistering a cursor

Change-Id: Ifab4705bc1cfe83417e4702dc9bac18ce45de5a4
Reviewed-on: http://review.couchbase.org/37392
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>