Handle Skill::sp_percent correctly

This field defaults to 0. When in the rm2k3 editor, you select the radio button to choose percent for the first time it immediately changes to 1 and can never be set to 0 again.

We don't necessarily have to do it this way, but having both `sp_type == percent && sp_percent == 0` is not a valid state.

Mentioning this as its a bit of an edge case to not forget.