Savegame slots over 64 are unusable

[walkawayy]

Investigated by Lahm:

So the limit is 64 slots, and it's because of this 
int32_t gf_param = gf_option & ((1 << 6) - 1);
e.g. stemming from return GF_START_SAVED_GAME | g_GameInfo.current_save_slot in the inventory
https://github.com/LostArtefacts/TR1X/blob/develop/src/game/shell.c#L187

Hit 65, and it loops back to slot 1.