Description:
------------
debug_backtrace crashes PHP if called from a class method after $this is set to null.

Reproduce code:
---------------
The following script illustrates the problem:

<?php

class Foo {
  function Bar() {
    $__this = $this;
//    unset($this);
    $this = null;
    print_r(debug_backtrace());
    $this = $__this;
  } 
}

$f = new Foo;

$f->Bar();

?>

Strangely, if you use unset instead of assigning $this to null, the crash does not occur.

Actual result:
--------------
Program received signal SIGSEGV, Segmentation fault.
0x4025ded4 in zif_debug_backtrace (ht=0, return_value=0x81a260c, this_ptr=0x0, return_value_used=1)
    at /usr/local/php/src/php4-STABLE-200307100130/Zend/zend_builtin_functions.c:1246
1246                                    class_name = ptr->object.ptr->value.obj.ce->name;
(gdb) bt
#0  0x4025ded4 in zif_debug_backtrace (ht=0, return_value=0x81a260c, this_ptr=0x0, return_value_used=1)
    at /usr/local/php/src/php4-STABLE-200307100130/Zend/zend_builtin_functions.c:1246
#1  0x40267f38 in execute (op_array=0x81a21fc) at /usr/local/php/src/php4-STABLE-200307100130/Zend/zend_execute.c:1616
#2  0x402680f6 in execute (op_array=0x819d434) at /usr/local/php/src/php4-STABLE-200307100130/Zend/zend_execute.c:1660
#3  0x40255940 in zend_execute_scripts (type=8, retval=0x0, file_count=3)
    at /usr/local/php/src/php4-STABLE-200307100130/Zend/zend.c:886
#4  0x4022e83c in php_execute_script (primary_file=0xbffff6e0)
    at /usr/local/php/src/php4-STABLE-200307100130/main/main.c:1685
#5  0x4026f34a in apache_php_module_main (r=0x81d9774, display_source_mode=0)
    at /usr/local/php/src/php4-STABLE-200307100130/sapi/apache/sapi_apache.c:54
#6  0x4026ff32 in send_php (r=0x81d9774, display_source_mode=0, filename=0x0)
    at /usr/local/php/src/php4-STABLE-200307100130/sapi/apache/mod_php4.c:620
#7  0x4026ff86 in send_parsed_php (r=0x81d9774)
    at /usr/local/php/src/php4-STABLE-200307100130/sapi/apache/mod_php4.c:635
#8  0x0808dd77 in ap_invoke_handler ()
#9  0x080a2e57 in process_request_internal ()
#10 0x080a2eb8 in ap_process_request ()
#11 0x08099cc9 in child_main ()
#12 0x08099e74 in make_child ()
#13 0x08099fe8 in startup_children ()
#14 0x0809a660 in standalone_main ()
#15 0x0809aec3 in main ()
#16 0x42017499 in __libc_start_main () from /lib/i686/libc.so.6