crash ns_heart if it's slow_updater child dies

Change-Id: I92c2740770f5b1f1ba76f14ec47a5ddd10fa8abf
Reviewed-on: http://review.couchbase.org/30350
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>