NCBC-192: Handle NRE when executing ops against down node

Change-Id: I2978a6507ead6b867b98f213405ebf486d252fd0
Reviewed-on: http://review.couchbase.org/23671
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: John C. Zablocki <john@couchbase.com>