Fixer mixer: Mixer Memory Leaks
Fix the memory leaks from the SDL2 Mixer interface. Mix_QuickLoad_RAW loaded Mix_Chunk's don't free the data used in Mix_FreeChunk since we allocated it. So Free it manually in I_FreeSfx. This means the allocation in ds2chunk can go back to being Z_Malloc. The SDL_RWops from SDL_RWFromMem need to be closed after being used.