[BUGIFX] Extra output block 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: 6.1, 6.0
Change-Id: I1d33bc3c5e8bbd1851f087c76564f8924f8dfe26
Reviewed-on: https://review.typo3.org/18959
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn