Flat changing hotfix

This should fix how Lua scripts that change flats in levels, particularly to flats that don't already exist in the level, cause SRB2 to crash for anyone who joins a netgame afterwards. (I'm not sure if it's a consistent thing or not though)

Probably needs a good test to verify the issue is completely fixed before being merged in. Maybe there are other minor things I didn't account for in this fix? I don't know!