[BUGIFX] Extra output blocks backend thumbnails

If some inclusion causes output when viewing image thumbnails
in the backend, the output will be broken. Adding a call to
ob_clean() ensures that this does not happen.

Resolves: #31953
Releases: 4.5, 4.7
Change-Id: I8fe3363bde1b63f7ae950397dd42e7b426a754b6
Reviewed-on: https://review.typo3.org/19004
Reviewed-by: Francois Suter
Tested-by: Francois Suter