grpcproxy: return nil on receiving snapshot EOF
grpcproxy: return nil on receiving snapshot EOF

Gets "code = OutOfRange desc = EOF" errors otherwise.

sinsharat/etcd,mkumatag/etcd,coreos/etcd,mkumatag/etcd,sebrandon1/etcd,glevand/coreos--etcd,joshix/etcd,etcd-io/etcd,akauppi/etcd,heyitsanthony/etcd,ajityagaty/etcd,lorneli/etcd,cockroachdb/etcd,li-ang/etcd,glevand/coreos--etcd,mqliang/etcd,coreos/etcd,ajityagaty/etcd,etcd-io/etcd,mickep76/etcd,bdarnell/etcd,YuleiXiao/etcd,dannysauer/etcd,YuleiXiao/etcd,mqliang/etcd,cockroachdb/etcd,gyuho/etcd,whuwxl/etcd,akauppi/etcd,mickep76/etcd,fanminshi/etcd,dannysauer/etcd,a-robinson/etcd,bluebreezecf/etcd,whuwxl/etcd,ajityagaty/etcd,li-ang/etcd,etcd-io/etcd,rnaveiras/etcd,rnaveiras/etcd,mqliang/etcd,fanminshi/etcd,bluebreezecf/etcd,sebrandon1/etcd,joshix/etcd,rnaveiras/etcd,sinsharat/etcd,a-robinson/etcd,vimalk78/etcd,xiang90/etcd,mkumatag/etcd,vimalk78/etcd,xiang90/etcd,visheshnp/etcd,bdarnell/etcd,visheshnp/etcd,lorneli/etcd,heyitsanthony/etcd,gyuho/etcd