clientv3: balancer panics when call up after close
clientv3: balancer panics when call up after close

Fix the issue by adding a simple guard varable.

ajityagaty/etcd,endocode/etcd,yudai/etcd,nekto0n/etcd,YuleiXiao/etcd,nekto0n/etcd,rchicoli/etcd,visheshnp/etcd,gyuho/etcd,endocode/etcd,etcd-io/etcd,rnaveiras/etcd,glevand/coreos--etcd,sinsharat/etcd,fanminshi/etcd,joshix/etcd,handsomesun/etcd,jlamillan/etcd,luxas/etcd,YuleiXiao/etcd,armstrongli/etcd,hongchaodeng/etcd,a-robinson/etcd,bluebreezecf/etcd,li-ang/etcd,insequent/etcd,handsomesun/etcd,xiang90/etcd,luxas/etcd,zaphod-concur/etcd,mkumatag/etcd,ajityagaty/etcd,ravigadde/etcd,armstrongli/etcd,lorneli/etcd,cockroachdb/etcd,CrazyJvm/etcd,lorneli/etcd,yudai/etcd,mickep76/etcd,tguangch/etcd,mkumatag/etcd,zaphod-concur/etcd,tguangch/etcd,gyuho/etcd,a-robinson/etcd,joshix/etcd,dannysauer/etcd,glevand/coreos--etcd,xiang90/etcd,akauppi/etcd,ajityagaty/etcd,vimalk78/etcd,coreos/etcd,qqsun8819/etcd,li-ang/etcd,whuwxl/etcd,mqliang/etcd,fanminshi/etcd,akauppi/etcd,hongchaodeng/etcd,mqliang/etcd,insequent/etcd,OyTao/etcd_learning,qqsun8819/etcd,etcd-io/etcd,etcd-io/etcd,plasticbox/etcd,rnaveiras/etcd,lumjjb/etcd,CrazyJvm/etcd,lumjjb/etcd,sinsharat/etcd,cockroachdb/etcd,bluebreezecf/etcd,bdarnell/etcd,rnaveiras/etcd,sebrandon1/etcd,visheshnp/etcd,vimalk78/etcd,jlamillan/etcd,mkumatag/etcd,dannysauer/etcd,whuwxl/etcd,heyitsanthony/etcd,heyitsanthony/etcd,ravigadde/etcd,sebrandon1/etcd,coreos/etcd,plasticbox/etcd,mqliang/etcd,rchicoli/etcd,OyTao/etcd_learning,mickep76/etcd,bdarnell/etcd