File loading hotfix

Fixes SRB2MB: Executing a TXT a lot makes the game no longer saves your config/data.

Turns out that when you attempt to add a file that's already loaded, the game forgets to close its handle. This means that if you add the file enough times you actually hit the limit for how many files you can have open eventually. The MB's thread way is just one way of getting this quickly it turns out; another one found by Steel is just having many lines of adding the same file redundantly in the text file, where you only need to run it once before you get the same bug.