Fix false positive error messages from console

loadPreview() is called within readGameSlot() and loadPortrait() within updateButtons() so we don't need these calls twice. This removes terminal messages

Couldn't load image: Couldn't open ./images/avatar/male/preview/dagger.png
Couldn't load image: Couldn't open ./images/avatar/male/preview/head_short.png

reported in https://github.com/makrohn/polymorphable/issues/34