Fix bug when cleaning visibility after subtree movement

Fix bug when cleaning visibility after subtree movement

Bernt Hansen writes:

> ,----[ test.org ]
> |
> | * Main task
> | ** DONE Task 1
> | ** DONE Task 2
> | ** TODO Task 4
> | ** TODO Task 3
> | ** TODO Task 5
> | * Next task
> `----
>
> Put the point on * Main Task
> C-x n s to narrow to subtree
> C-c C-v to show active TODOs
> Put point on TODO Task 4
> M-down arrow
> M-up arrow
>
> generates
>
> Debugger entered--Lisp error: (error "before first heading")