MB-14098: Check to prevent roll over while decrementing running backfill count
MB-14098: Check to prevent roll over while decrementing running backfill count

Do not decrement the running backfill count when it is zero.
It is safer to have this check though we have not proved this is causing any
problems as of now.
Change-Id: Ie17beb5b7432ad2892e84f2b683a3efd978eb0ae
Reviewed-on: http://review.couchbase.org/48818
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>
Reviewed-by: Michael Wiederhold <a17fed27eaa842282862ff7c1b9c8395a26ac320@couchbase.com>
Tested-by: Manu Dhundi <158873d90a7ef40f3637a222b7329c09d0222554@couchbase.com>

sriganes/ep-engine,daverigby/kv_engine,hisundar/ep-engine,hisundar/ep-engine,daverigby/ep-engine,sriganes/ep-engine,membase/ep-engine,owendCB/ep-engine,owendCB/ep-engine,owendCB/ep-engine,membase/ep-engine,couchbase/ep-engine,hisundar/ep-engine,sriganes/ep-engine,couchbase/ep-engine,teligent-ru/ep-engine,couchbase/ep-engine,daverigby/kv_engine,teligent-ru/ep-engine,owendCB/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,daverigby/kv_engine,hisundar/ep-engine,couchbase/ep-engine,membase/ep-engine,membase/ep-engine,daverigby/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,sriganes/ep-engine,daverigby/kv_engine