[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
Reviewed-on: https://review.typo3.org/19004
Reviewed-by: Francois Suter
Tested-by: Francois Suter
Change-Id: Ic540939d2a39b0adeb4427a6e772ad506686378d
Reviewed-on: https://review.typo3.org/19005