MB-10828: Handling a possible segmentation fault

- If vb is NULL, there could be a segmentation
fault when using the object to retrieve the vbucket
hash table, etc.
- Return if vb is NULL

Change-Id: Ic646a529f9ca03b170629a43edb3fc3dd717c8d6
Reviewed-on: http://review.couchbase.org/35891
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>