fix panic 5435 (#5436)

weed/server/master_grpc_server_assign.go

@@ -97,6 +97,9 @@ func (ms *MasterServer) Assign(ctx context.Context, req *master_pb.AssignRequest
 			continue
 		}
 		dn := dnList.Head()
+		if dn == nil {
+			continue
+		}
 		var replicas []*master_pb.Location
 		for _, r := range dnList.Rest() {
 			replicas = append(replicas, &master_pb.Location{

weed/server/master_server_handlers.go

@@ -149,7 +149,9 @@ func (ms *MasterServer) dirAssignHandler(w http.ResponseWriter, r *http.Request)
 		} else {
 			ms.maybeAddJwtAuthorization(w, fid, true)
 			dn := dnList.Head()
-
+			if dn == nil {
+				continue
+			}
 			writeJsonQuiet(w, r, http.StatusOK, operation.AssignResult{Fid: fid, Url: dn.Url(), PublicUrl: dn.PublicUrl, Count: count})
 			return
 		}