MB-3652 Don't close the checkpoint that has been already closed.
MB-3652 Don't close the checkpoint that has been already closed.

Change-Id: I6124fbc9519bed582816a8613bb4259a5d0448e6
Reviewed-on: http://review.membase.org/5538
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

couchbaselabs/ep-engine,owendCB/ep-engine,teligent-ru/ep-engine,hisundar/ep-engine,sriganes/ep-engine,sriganes/ep-engine,membase/ep-engine,zbase/ep-engine,owendCB/ep-engine,couchbaselabs/ep-engine,zbase/ep-engine,teligent-ru/ep-engine,sriganes/ep-engine,daverigby/ep-engine,couchbaselabs/ep-engine,membase/ep-engine,owendCB/ep-engine,daverigby/ep-engine,hisundar/ep-engine,zbase/ep-engine,membase/ep-engine,abhinavdangeti/ep-engine,teligent-ru/ep-engine,daverigby/kv_engine,daverigby/ep-engine,sriganes/ep-engine,membase/ep-engine,daverigby/kv_engine,couchbaselabs/ep-engine,teligent-ru/ep-engine,daverigby/ep-engine,couchbaselabs/ep-engine,hisundar/ep-engine,abhinavdangeti/ep-engine,daverigby/kv_engine,abhinavdangeti/ep-engine,daverigby/kv_engine,jimwwalker/ep-engine,abhinavdangeti/ep-engine,abhinavdangeti/ep-engine,zbase/ep-engine,couchbase/ep-engine,jimwwalker/ep-engine,couchbase/ep-engine,couchbase/ep-engine,owendCB/ep-engine,couchbase/ep-engine,zbase/ep-engine,jimwwalker/ep-engine,jimwwalker/ep-engine,hisundar/ep-engine