MB-11433 fix obvious arithmetic mistake

Change-Id: I05186f41aba75cfed7056e551a7d3adf114be0ba
Reviewed-on: http://review.couchbase.org/38841
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>