Only remove oldChild if it's not null

Although the comment next to this code says you should only fall into this branch if you're not reusing views correctly, I believe I am and am landing there because I'm using the drag-sort-listview with a [`MergeAdapter`](https://github.com/commonsguy/cwac-merge) containing some static views. This pull request fixes a crash that occurs if the row to be reused doesn't have any children, which is posible in my situation.