MB-11158: check for NULL stream in getNextItem() of UPR consumer

While retrieving the next item on the UPR consumer, add a NULL check
while accessing a stream for a particular vbucket.

Change-Id: I429337a0194afbe9fd7ee2786a75287498504823
Reviewed-on: http://review.couchbase.org/37306
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>