Changed the order of deleting zone trees.

- NSEC3 tree must be deleted before the normal tree, as the RDATA
  in it may contain domain names from the normal tree and checking
  them would otherwise result in undefined behaviour (as those
  names will already be deleted).

src/dnslib/zone.c

@@ -988,17 +988,19 @@ void dnslib_zone_deep_free(dnslib_zone_t **zone)
 #endif
 	/* has to go through zone twice, rdata may contain references to node
 	   owners earlier in the zone which may be already freed */
+	/* NSEC3 tree is deleted first as it may contain references to the
+	   normal tree. */
 
-	TREE_POST_ORDER_APPLY((*zone)->tree, dnslib_node, avl,
+	TREE_POST_ORDER_APPLY((*zone)->nsec3_nodes, dnslib_node, avl,
 	                      dnslib_zone_destroy_node_rrsets_from_tree, NULL);
 
- 	TREE_POST_ORDER_APPLY((*zone)->tree, dnslib_node, avl,
+	TREE_POST_ORDER_APPLY((*zone)->nsec3_nodes, dnslib_node, avl,
 	                      dnslib_zone_destroy_node_owner_from_tree, NULL);
 
-	TREE_POST_ORDER_APPLY((*zone)->nsec3_nodes, dnslib_node, avl,
+	TREE_POST_ORDER_APPLY((*zone)->tree, dnslib_node, avl,
 	                      dnslib_zone_destroy_node_rrsets_from_tree, NULL);
 
-	TREE_POST_ORDER_APPLY((*zone)->nsec3_nodes, dnslib_node, avl,
+ 	TREE_POST_ORDER_APPLY((*zone)->tree, dnslib_node, avl,
 	                      dnslib_zone_destroy_node_owner_from_tree, NULL);
 
 	dnslib_zone_free(zone);