Fix bug in CSV table import

Fix bug in CSV table import

Dan Davison writes:

> If a file contains "-1" followed by a newline and nothing else,
> org-table-import on that file fails. The first commit with this
> property is a commit (below) to do with CVS tables made a few
> days ago. I have given up trying to work out a good solution to
> this :) In case it is useful, the failure occurs when
> org-table-align is called at the end of
> org-table-convert-region. I think it is long-standing behaviour
> that hitting tab inside of
>
> |-1|
>
> doesn't make a table containing "-1", so presumably there is
> something different about the context in which org-table-align is
> now being called.