backend: skip *bolt.DB.Size call when nil
backend: skip *bolt.DB.Size call when nil

Fix https://github.com/coreos/etcdlabs/issues/30.

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