improve iterating a list to fail fast in a children binding

Steps to Reproduce

1. run the attached example
2. compare clicking "index" button and other 2 buttons (for, iterator)

Current Result

• "index" doesn't always cause an exception
• for & iterator always throws ConcurrentModificationException

Expected Result

implement with a loop that failed fast is better to expose the root cause, so that application developers can fix their problem eariler.

Debug Information

• at at org.zkoss.bind.impl.LoadChildrenBindingImpl.load(LoadChildrenBindingImpl.java:121)