[BUGFIX] additionalAttributes for be.buttons.icon-VH misses hsc

The values of the additional attributes for the backend icon viewhelper
need to be processed through htmlspecialchars().

Resolves: #53711
Releases: 6.2, 6.1, 6.0
Change-Id: I89794c77ad1bb7bad99517e24ae7345e0803616e
Reviewed-on: https://review.typo3.org/25485
Reviewed-by: Marc Bastian Heinrichs
Tested-by: Marc Bastian Heinrichs