followup r79862: the for loop only cleans up half the output handlers (since $i count...

followup r79862: the for loop only cleans up half the output handlers (since $i counts up while ob_get_level() counts down); check the return value of ob_end_clean() instead.
(I just noticed this while eyeballing the code -- apparently most people don't have multiple output handlers active, given that nobody had caught this in over eight months.)