Fixes for ArsModelDownloads

Some fixes for ArsModelDownloads. I'm not sure if Netbeans has such an awesome error validating feature as PHPStorm, but all these issues were spotted within minutes with PHPStorm  (and would be prevented in the first place ;). The undefined variables are underlined in red.

This likely also fixes this support ticket:
https://www.akeebabackup.com/support/akeeba-release-system/17087-blank-item-page.html

Cheers mate!