zone: correct lookup of previous node, which must be non-empty and

authoritative

- this is temporary, as looking up closest encloser and previous
  is not required (or doesn't make sense) in all situations
- zone search should always return lexicographically previous node
  and the proof code should look up the encloser or previous node
  should it be needed

src/knot/zone/zone-tree.c

@@ -166,8 +166,8 @@ int knot_zone_tree_get_less_or_equal(knot_zone_tree_t *tree,
 		hattrie_iter_free(i);
 	}
 
-	/* Check if previous node is not an empty non-terminal. */
-	if (knot_node_rrset_count(*previous) == 0) {
+	/* Previous node for proof must be non-empty and authoritative. */
+	if (knot_node_rrset_count(*previous) == 0 || knot_node_is_non_auth(*previous)) {
 		*previous = knot_node_get_previous(*previous);
 	}