add rbtree_node_t virtual destructor, possible memory leak

Author: xtaci

include/rbtree_defs.h

@@ -35,6 +35,8 @@ namespace alg
 rbtree_node_t* right;
 rbtree_node_t* parent;
 enum rbtree_node_color color;
+virtual ~rbtree_node_t() {
+}
 } *rbtree_node;
 
 private: