Fix obvious pasto in PostOrder walk.
Use a consistent callback return code

  This is the one place in the codebase where a positive return code
  is used in an rbtree_walk callback.  Changing it negative should have no
  effect, and keeps options open as to whether to introduce new symbols
  for the next patch to follow, or to change the libfreeradius API
  slightly WRT rbtree callback return codes.

  Whether any third-party add-ons may use positive return codes is not
  known.
Always fixup after deleting a black child node in rbtree

	This prevents rbtree_delete from creating unbalanced/illegal rbtrees.