Check for bucket and vbucket existence in EvpGetItemInfo

Change-Id: I26a4bf6be5955475e6cfd57b14727be953d8aa0b
Reviewed-on: http://review.couchbase.org/44636
Tested-by: Sriram Ganesan <sriram@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>