MB-9044: fixup check for stats.json existence

Change-Id: I1a02ed27eda5d2bd46a3b7431952c45d3c2dc0ef
Reviewed-on: http://review.couchbase.org/28784
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>