CBD-1425:  fix logic error in last_done_server()

Change-Id: I02f278e267e8bd4364aeaf6bb00732738a54b4a2
Reviewed-on: http://review.couchbase.org/41964
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>