Description:
------------
Bug actual for 7.1.9, 7.2.0beta2 - 7.2.0rc1
Not reproduced in 5.6.0 - 5.6.30, hhvm-3.18.4 - 3.21.0, 7.0.0 - 7.1.8, 7.2.0alpha1 - 7.2.0beta1
Online comparsion https://3v4l.org/OMDFY

My application crashed after update to 7.1.9



Test script:
---------------
$a = new \AppendIterator();
$a->append(new \ArrayIterator([1]));
    
$b = new \AppendIterator();
$b->append($a);
$b->append(new \ArrayIterator([1]));
    
foreach ($b AS $z){
    var_dump($z);
}

Expected result:
----------------
int(1)
int(1)

Actual result:
--------------
object(ArrayIterator)#4 (1) {
  ["storage":"ArrayIterator":private]=>
  array(1) {
    [0]=>
    int(1)
  }
}
int(1)