MB-10476: Use cb_assert which is never a noop

Change-Id: Ibf478e0dc55697ea7b9fe1aff1ac18a4ec1ea0c7
Reviewed-on: http://review.couchbase.org/34580
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: buildbot <build@couchbase.com>