rb.h: macro RB_FOR_EACH_CONTAINER bug

[changkong5]

Describe the bug
The macro RB_FOR_EACH_CONTAINER in the rb.h when the function _rb_foreach_next(&tree, &__f) return NULL then the CONTAINER_OF will retrun error.

example:

   #define MAX_NODES 8
   struct rbextnode {
	int key;
	int val;
	struct rbnode node;
   } nodes[MAX_NODES];

   static struct rbtree tree;
   struct rbextnode *extnode;

   ......
   RB_FOR_EACH_CONTAINER(&tree, extnode, node) {
      ...
   }
   ......

Test on nrf52832 board when the function _rb_foreach_next(&tree, &__f) in the macro RB_FOR_EACH_CONTAINER return NULL then the variable extnode is 0xFFFFFFF8 and run forever in for structural body.

To Reproduce