Skip AV scanning on non-theme scans

Only Theme scans support ClamAV currently.

See #28
Skip vip-init.php check on non-theme scans

Since we don't have the context of the entire theme, we can't
accurately detect if we're looking at the main functions.php, so skip
this check on non-theme scans, such as diff scans
Fix the class check in get_path()

get_path() is a method of ThemeScanner, not BaseScanner