Description:
------------
This iterator causes a crash due to recursion.


Test script:
---------------
<?php

$iterator = new AppendIterator(array("A","A","A"));
$iterator->append($iterator);
?>


Expected result:
----------------
ho crash 

Actual result:
--------------
gdb-peda$ r
Starting program: /home/hjy/Desktop/php-7.1.7/sapi/cli/php -n poc.php
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/i386-linux-gnu/libthread_db.so.1".

Program received signal SIGSEGV, Segmentation fault.

[----------------------------------registers-----------------------------------]
EAX: 0x8d1bfe8 --> 0x1 
EBX: 0x0 
ECX: 0xb6e6d2b8 --> 0x6 
EDX: 0x8d1bdc0 --> 0x1 
ESI: 0xb6e14020 --> 0xb6e5e3a8 --> 0x854992f (<ZEND_DO_FCALL_SPEC_RETVAL_UNUSED_HANDLER>:	push   ebp)
EDI: 0xb6e5e3a8 --> 0x854992f (<ZEND_DO_FCALL_SPEC_RETVAL_UNUSED_HANDLER>:	push   ebp)
EBP: 0xbf800018 
ESP: 0xbf7fffe0 
EIP: 0x84e098b (<zend_vm_stack_push_call_frame+9>:	mov    DWORD PTR [esp+0x4],eax)
EFLAGS: 0x210282 (carry parity adjust zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x84e0983 <zend_vm_stack_push_call_frame+1>:	mov    ebp,esp
   0x84e0985 <zend_vm_stack_push_call_frame+3>:	sub    esp,0x38
   0x84e0988 <zend_vm_stack_push_call_frame+6>:	mov    eax,DWORD PTR [ebp+0xc]
=> 0x84e098b <zend_vm_stack_push_call_frame+9>:	mov    DWORD PTR [esp+0x4],eax
   0x84e098f <zend_vm_stack_push_call_frame+13>:	
    mov    eax,DWORD PTR [ebp+0x10]
   0x84e0992 <zend_vm_stack_push_call_frame+16>:	
    mov    DWORD PTR [esp],eax
   0x84e0995 <zend_vm_stack_push_call_frame+19>:	
    call   0x84e092d <zend_vm_calc_used_stack>
   0x84e099a <zend_vm_stack_push_call_frame+24>:	
    mov    DWORD PTR [ebp-0xc],eax
[------------------------------------stack-------------------------------------]
Invalid $SP address: 0xbf7fffe0
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Stopped reason: SIGSEGV
0x084e098b in zend_vm_stack_push_call_frame (call_info=0x202, func=0x8d1bfe8, 
    num_args=0x0, called_scope=0x8d1bdc0, object=0xb6e6d2b8)
    at /home/hjy/Desktop/php-7.1.7/Zend/zend_execute.h:209
209		uint32_t used_stack = zend_vm_calc_used_stack(num_args, func);