[BUGFIX] Add missing htmlspecialchars for thumbnail URL

Resolves: #56057
Releases: 6.2, 6.1
Change-Id: I9c1ec60e7518867d27e99681d3d18957a27c4983
Reviewed-on: https://review.typo3.org/27670
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel