[SDL Audio] Refactoring Mix_LoadWAV_RW to use new FS API.

The old API was causing issues with certain file systems (well, my custom NODEFS-based file system), as the `read` property required the execute bit AND the read bit to be set before it would return `true`.

As a side benefit, the new code is simpler and cleaner.

Fixes https://github.com/kripken/emscripten/issues/1893