* nih/tests/test_tree.c (test_next, test_prev, test_next_pre)
(test_prev_pre, test_next_post, test_pre_post): Add tests for a
single-node tree, because we found a bug where it didn't work.
* nih/tree.c (nih_tree_next): Bug fix; check there is a right node
before checking whether we've returned from it, otherwise we could
confuse it with starting at the top of the tree.
(nih_tree_prev): Likewise check there is a left node before checking
whether we've returned from it.