Fix the filesystem implementation in order to get blargSnes to run

Well, as far as I can see the only point in keeping a variable around is for caching, i.e. performance reasons. However, I'd expect the iterator returned by `.end()` to be cached anyway internally, but even if that's not the case I don't think it's an issue we need to worry about unless it actually shows up high in profiling.

On the other hand, while it's probably not too much of an issue in this particular case, keeping iterators around for too long might cause them to point to invalid data eventually iff the container is modified.

So yeah, better to retrieve it every time I guess.