in certain cases hmbkp_default_path can have incorrect value

If the site home path is changed, the value of hmbkp_default_path will not contain the correct path, thus resulting in 404 errors when trying to download a backup file.

I traced this back to the `hmbkp_path_default` function in core.php

- [x] add extra check to see if it is still valid, otherwise update it

related: https://github.com/humanmade/backupwordpress/issues/298

I wonder if this is related to the recent outage that Hostgator experienced?