[BUGFIX] Prevent XSS in scheduler extension

Releases: master,8.7,7.6
Resolves: #82079
Change-Id: I21a6ebcff4ebd0c6f2d2c83e0aa6d9e2c03d32d9
Reviewed-on: https://review.typo3.org/53720


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wolfgang Klinger <wolfgang@wazum.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>