[libvirt] hash.c patches

Hi, I recently started looking at libvirt code, and while reading hash.c, I noticed a few buglets and some cleanup that could be done. I was planning to test them carefully before sending them (ie make sure there's a unit test for this code), but since I don't know when I have time for that, and since make check and make syntax-check are passing, I'm just sending them for an initial review :) If you prefer the patches inline/in separate mails, just let me know. Thanks! Christophe