Allow Cosmetic Add-Ons Mid-Save

Fixes #911 (closed), thanks to @ SteelT's analysis.
Keeping the cursaveslot > 0 check is necessary, since it's set to -1 when viewing the credits through the Extras menu, and the credits check if it's -1 to skip the "you beat the game" evaluation. If cursaveslot was always set to 0, you could "beat" the game by adding an add-on mid-credits.

TL;DR:
Loading cosmetic add-ons mid-game no longer stops saving the current save slot