rlp: finalize listIterator on parse error to prevent non-advancing loops

[radik878]

The list iterator previously returned true on parse errors without advancing the input, which could lead to non-advancing infinite loops for callers that do not check Err() inside the loop; to make iteration safe while preserving error visibility, Next() now marks the iterator as finished when readKind fails, returning true for the error step so existing users that check Err() can handle it, and then false on subsequent calls, and the function comment was updated to document this behavior and the need to check Err().