pctrie: leave pctrie top as current after [gl]e_lookup failure

[dougm]

Summary

If pctrie_lookup_iter_ge fails to return a node, the iterator is left with NULL as the current node. Instead, make the top of the tree the current node. Do the same thing for lookup_iter_le. If an iterator is reused after a ge/le lookup fails, this will skip the step in _pctrie_lookup_node where a NULL is replaced by the node at the top of the trie.

[dougm]

Handle the case where the pctrie is empty.