fix magic_exp not updating last used spell index

Created by: ewewukek

I've noticed that casting #plugh and #miteyo alternately did not update #abizit message after more than 150 #plugh casts. I've found that it is due to magic_exp function returning without updating last used spell index for spells with zero exp_gain.

If this behavior is intended and one is not allowed to level up magic repeating spells with zero and non-zero cost, please leave a comment and close this PR :)