Prevent 'Use of uninitialized value' when out is already flushed.

On perl 5.10.1 i get a lot of warnings when using [Rex](https://github.com/RexOps/Rex) to upload files, and it looks like I'm not the only one: RexOps/Rex#199 

```shell
Use of uninitialized value in length at /Users/djo/.plenv/versions/5.10.1/lib/perl5/site_perl/5.10.1/Net/SFTP/Foreign.pm line 758.
```

It looks like the warning is generated when there is nothing to flush or out has already been flushed.