Description:
------------
Prior to 5.4.0RC1, when calling ob_get_clean() with no buffers present, no errors 
were raised. Starting with 5.4.0RC1, the function now raises an E_NOTICE in such 
situations.

Test script:
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ini_set('display_errors', true);                                                
error_reporting(-1);                                                            
$return = ob_get_clean();

Expected result:
----------------
(no output)

Actual result:
--------------
Notice: ob_get_clean(): failed to delete buffer. No buffer to delete