grpcproxy: return nil on receiving snapshot EOF

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

proxy/grpcproxy/maintenance.go

@@ -15,6 +15,8 @@
 package grpcproxy
 
 import (
+	"io"
+
 	"golang.org/x/net/context"
 
 	"github.com/coreos/etcd/clientv3"
@@ -49,6 +51,9 @@ func (mp *maintenanceProxy) Snapshot(sr *pb.SnapshotRequest, stream pb.Maintenan
 	for {
 		rr, err := sc.Recv()
 		if err != nil {
+			if err == io.EOF {
+				return nil
+			}
 			return err
 		}
 		err = stream.Send(rr)