Show unknown.png regardless of PARAM.SFO when an ISO lacks an icon0.

It seems weird to show only a black rectangle that has nothing in it (and nothing when viewing the right-click details page) when certain ISOs (e.g. homebrew ISOs, or broken ISOs) have no icon0, so let's show the unknown.png file instead.

![screenshot 01](http://i.minus.com/itemKPShLJkhR.jpg)

![screenshot 02](http://i.minus.com/iyoDNOmxHCLGk.jpg)