Description:
------------
Tested on 5.2.0RC6

Unsetting a static variable referring to the object itself causes a segfault later on. (possible alloc problems)

I was able to reproduce segfaults in this situation with other functions besides debug_backtrace(), for instance with mysqli_fetch_assoc(). The resulting backtrace also led to  _zend_mm_alloc_int. (I am presuming it is the same bug)

PS. The print_r() is not required to trigger the crash.

Reproduce code:
---------------
<?php
class test
{
	protected $_id;
	static $instances;
	
	public function __construct($id)
	{
		$this->test();
			
		$this->_id = $id;
		
		self::$instances[$this->_id] = $this;
	}
	
	function __destruct()
	{
		unset(self::$instances[$this->_id]);
	}
	
	function test()
	{
		print_r(debug_backtrace());	
	}
	
}
	
	$test = new test(2);
	
	$test = new test(1);
	
	$test = new test(2);

	$test = new test(3);
?>

Expected result:
----------------
No crash.

Actual result:
--------------
#0  _zend_mm_alloc_int (heap=0x80ebbb8, size=16) at /usr/src/php-5.2.0RC6/Zend/zend_alloc.c:1090 
#1  0x4063f953 in add_assoc_long_ex (arg=0x3, key=0x40769a60 "line", key_len=5, n=16) at /usr/src/php-5.2.0RC6/Zend/zend_API.c:977 
#2  0x4064d2d8 in zend_fetch_debug_backtrace (return_value=0x40f289cc, skip_last=1, provide_object=1) 
    at /usr/src/php-5.2.0RC6/Zend/zend_builtin_functions.c:1962 
#3  0x40658d54 in zend_do_fcall_common_helper_SPEC (execute_data=0xbfffacc0) at /usr/src/php-5.2.0RC6/Zend/zend_vm_execute.h:200 
#4  0x40658489 in execute (op_array=0x40f282c8) at /usr/src/php-5.2.0RC6/Zend/zend_vm_execute.h:92 
#5  0x40658709 in zend_do_fcall_common_helper_SPEC (execute_data=0xbfffae80) at /usr/src/php-5.2.0RC6/Zend/zend_vm_execute.h:234 
#6  0x40658489 in execute (op_array=0x40f28fd4) at /usr/src/php-5.2.0RC6/Zend/zend_vm_execute.h:92 
#7  0x40658709 in zend_do_fcall_common_helper_SPEC (execute_data=0xbfffb0e0) at /usr/src/php-5.2.0RC6/Zend/zend_vm_execute.h:234 
#8  0x40658489 in execute (op_array=0x40f24194) at /usr/src/php-5.2.0RC6/Zend/zend_vm_execute.h:92 
#9  0x4063ebfc in zend_execute_scripts (type=8, retval=0x0, file_count=3) at /usr/src/php-5.2.0RC6/Zend/zend.c:1097 
#10 0x40604e2a in php_execute_script (primary_file=0xbfffd440) at /usr/src/php-5.2.0RC6/main/main.c:1758 
#11 0x406bf882 in apache_php_module_main (r=0x80cb5bc, display_source_mode=0) at /usr/src/php-5.2.0RC6/sapi/apache/sapi_apache.c:53 
#12 0x406c0296 in send_php (r=0x80cb5bc, display_source_mode=0, filename=0x0) at /usr/src/php-5.2.0RC6/sapi/apache/mod_php5.c:660 
#13 0x406c04a6 in send_parsed_php (r=0x80cb5bc) at /usr/src/php-5.2.0RC6/sapi/apache/mod_php5.c:675 
#14 0x08053ff7 in ap_invoke_handler () 
#15 0x08069039 in process_request_internal () 
#16 0x08069098 in ap_process_request () 
#17 0x080600ba in child_main () 
#18 0x08060262 in make_child () 
#19 0x080603c8 in startup_children () 
#20 0x08060a88 in standalone_main () 
#21 0x080612a6 in main ()