Fix memleak in nathelper natping when "failed to fetch contacts".

Re-ordered some code around in nh_timer() so buf is always freed. Even
in rare cases when ul.get_all_ucontacts() returns false (because someone
broke the database).
Merge pull request #68 from wdoekes/wjd-fix_nathelper_memleak

Fix memleak in nathelper natping when "failed to fetch contacts".