Don't write a lockfile unless there's an actual file open (not just an in-memory buffer).

Original nano commut: 4584
Original nano commit message:
2014-01-25 Chris Allegretta <chrisa@asty.org>
        * src/winio.c (set_modified) - Check for a filename before we bother trying to lock
Rename 'nano.texi' to 'pinot.texi'
Update pinot.texi