Use hasOwnProperty in Network.prototype._isMoving

lib/network/Network.js

@@ -2324,7 +2324,7 @@ Network.prototype._restoreFrozenNodes = function() {
 Network.prototype._isMoving = function(vmin) {
   var nodes = this.nodes;
   for (var id in nodes) {
-    if (nodes[id] !== undefined) {
+    if (nodes.hasOwnProperty(id)) {
       if (nodes[id].isMoving(vmin) == true) {
         return true;
       }