[ticket/10191] Use ob_get_level in exit_handler for output buffering check.

Calling flush() when output buffering is enabled causes output to be
duplicated. Besides phpBB enabling output buffering for gzip compression,
output buffering may be enabled externally to phpBB via output_handler
or output_buffering directives in php.ini.

Use ob_get_level to determine whether output buffering is active and
call ob_flush in that case.

PHPBB3-10191

phpBB/includes/functions.php

@@ -4719,7 +4719,7 @@ function exit_handler()
 	}
 
 	// As a pre-caution... some setups display a blank page if the flush() is not there.
-	(empty($config['gzip_compress'])) ? @flush() : @ob_flush();
+	(ob_get_level() > 0) ? @ob_flush() : @flush();
 
 	exit;
 }