Fix leak while checking memory thresholds during warmup

Change-Id: I13afce24fe325d334575d7d8e315ebe46aa23db7
Reviewed-on: http://review.couchbase.org/40085
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>