[BUGFIX] Fix breaking t3editor by using hsc()

t3editor misses a htmlspecialchars() when displaying content of
the CE "HTML".

Change-Id: I84b65cd42bee971adc6fb77149f6789e76fc6567
Fixes: #33651
Releases: 6.1,6.0,4.7,4.6,4.5
Reviewed-on: https://review.typo3.org/19584
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn