MB-100: minor fix in StatsWaitTask

Change-Id: I3933535d0ff6238d6c9fe8d5e364a6554d38b055
Reviewed-on: http://review.couchbase.org/24380
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>