Description:
------------
When setting an output buffer that returns an empty string (I do have a good reason for doing this), later calls to ob_flush() cause PHP to seg fault. 

Tested on PHP-4.1.1 on linux (CLI and Apache2 module) with same error on both.

Also tested on PHP-5.0.5 on Win32 (CLI and Apache2 module) with same error on both.

Reproduce code:
---------------
<?php 
function nooutput($text) { return ""; } 
print "Before\n"; 
ob_start("nooutput"); 
print "Middle\n"; 
ob_flush();
ob_end_flush(); 
print "After\n"; 
?> 


Expected result:
----------------
Before
After

Actual result:
--------------
Before
Segmentation fault