MB-10240: Check vbucket existence in ItemResidentCallback

We need to make sure the vbucket exists before trying to lookup
and item in the cache.

Change-Id: I721b6333fa59d70688facf762ef35d2d78f7fd36
Reviewed-on: http://review.couchbase.org/34386
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Reviewed-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Tested-by: buildbot <build@couchbase.com>