Fix https://github.com/chrislusf/seaweedfs/issues/825#issuecomment-469097375

weed/server/master_grpc_server.go

@@ -102,7 +102,9 @@ func (ms *MasterServer) SendHeartbeat(stream master_pb.Seaweed_SendHeartbeatServ
 
 		// tell the volume servers about the leader
 		newLeader, err := t.Leader()
-		if err == nil {
+		if err != nil {
+			return err
+		}
 		if err := stream.Send(&master_pb.HeartbeatResponse{
 			Leader: newLeader,
 		}); err != nil {
@@ -110,7 +112,6 @@ func (ms *MasterServer) SendHeartbeat(stream master_pb.Seaweed_SendHeartbeatServ
 		}
 	}
 }
-}
 
 // KeepConnected keep a stream gRPC call to the master. Used by clients to know the master is up.
 // And clients gets the up-to-date list of volume locations