Check if a placeholder and its parent node exist before removing

These changes ensure that we take the same approach in `_clear`, as we do for [`cancel`](https://github.com/jquery/jquery-ui/blob/master/ui/jquery.ui.sortable.js#L463-467), ensuring that a placeholder does indeed exist before trying to traverse up to its parent and finally remove it.

FYI I have already signed the CLA.

I hope this is helpful.

Andreas