[BUGFIX] Better positionName extraction in executePositionedStringInsertion

Limit string explosion so that "LLL:EXT:" notation in $position
does not get broken up.

Resolves: #75830
Releases: master, 7.6
Change-Id: I13cc15562bd790040c3e45c9eb27ff029311c78f
Reviewed-on: https://review.typo3.org/47832


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>