[Debug] work-around https://bugs.php.net/61272

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8703
| License       | MIT

https://bugs.php.net/61272 is the real offender.
Using ob_end_flush() instead of ob_end_clean() is the fix.
As locally I don't want to know anything about failSafeHandle(), several ob_start() could have been started somewhere else. This loop just closes all these potentially opened buffers. Fail-safe and defensive coding practice.