[physfs] Seeking backwards in .pk3 archive files fails on OS X (works on Windows)

[Ryan C. Gordon]

> One of our developers on Windows does not have this crash.. seeking back
> to 0 in the archive-sourced file works fine for him. So I can only
> conclude that seeking backwards in zip-sourced files does not work
> correctly on OS X.

Can you see if this fixes it?

https://hg.icculus.org/icculus/physfs/rev/090cb9c74b87

(I'm not sure if you meant the head of the stable-2.0 branch, or 
literally the tip...this fix doesn't relate to what's going to be 2.1).

--ryan.