Save states fail in RetroArch due to missing "system/np2kai" directory

[n-rook]

Thanks for making this core available!

I was recently experimenting with the np2kai core in RetroArch. I wanted to use save states, but when I used them, RetroArch returned an error telling me np2kai didn't support save states.

When I enabled logging, I saw these errors:

[libretro WARN] Couldn't open file (W) C:\RetroArch-Win64\system\np2kai\temp_.sxx.
[libretro WARN] statsave_save_d failed -1[INFO] Core does not support save states.

I created this np2kai directory, and after that, save states began to work again.

I think the fix to this bug is simple: either save to a different directory, or create the "np2kai" directory before writing the temporary save state.