Fix list bug at beginning of buffer

Fix list bug at beginning of buffer

David Maus writes:

> When `org-previous-item' is called on an item with nothing above it
> Orgmode enters an infinite loop. The reason is that
> `org-previous-item' searches for non-empty lines by moving point up
> line by line and if there is nothing above an item point gets stuck on
> begin of buffer.
>
> example.org
> ,----
> |
> |  - Item
> `----
>
> Move point on Item, M-x org-previous-item RET and Orgmode enters the
> infinite loop.
>
> Attached patch adds a conditional clause to `org-previous-item' that
> leaves the search loop if point reaches beginning of buffer.