Description:
------------
Assignment by reference to property may lose reference.

Reproduce code:
---------------
<?php
$a = new stdClass; 
$a->x0 = new stdClass;
$a->x0->y0 = 'a';
$a->x0->y1 =& $a->x0;
$a->x0->y2 =& $a->x0;
$a->x0->y0 = 'b';
var_dump($a);
?>

Expected result:
----------------
object(stdClass)#1 (1) {
  ["x0"]=>
  &object(stdClass)#2 (3) {
    ["y0"]=>
    string(1) "b"
    ["y1"]=>
    &object(stdClass)#2 (3) {
      ["y0"]=>
      string(1) "b"
      ["y1"]=>
      *RECURSION*
      ["y2"]=>
      *RECURSION*
    }
    ["y2"]=>
    &object(stdClass)#2 (3) {
      ["y0"]=>
      string(1) "b"
      ["y1"]=>
      *RECURSION*
      ["y2"]=>
      *RECURSION*
    }
  }
}


Actual result:
--------------
object(stdClass)#1 (1) {
  ["x0"]=>
  object(stdClass)#2 (3) {
    ["y0"]=>
    string(1) "b"
    ["y1"]=>
    object(stdClass)#2 (3) {
      ["y0"]=>
      string(1) "b"
      ["y1"]=>
      *RECURSION*
      ["y2"]=>
      *RECURSION*
    }
    ["y2"]=>
    object(stdClass)#2 (3) {
      ["y0"]=>
      string(1) "b"
      ["y1"]=>
      *RECURSION*
      ["y2"]=>
      *RECURSION*
    }
  }
}
/home/dmitry/php/php5.1/Zend/zend_vm_execute.h(393) :  Freeing 0x08B29C94 (16 bytes), script=ass4.php
=== Total 1 memory leaks detected ===