[BUGFIX] Make DataHandler multibyte-safe

There are still a few places left where multibyte string functions
are not used. Since 8.x the core delivers a mbstring polyfill
and can be used in various places.

Resolves: #19774
Releases: master, 8.7
Change-Id: I16f022b109749e9f547c177c511fee3bcc5d4b60
Reviewed-on: https://review.typo3.org/52752


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>