[BUGFIX] Check whether contentElementCache for language and colPos is valid

It may happen that ``$this->contentElementCache[$lP][$colPos]`` is either not
an array nor null. Wrap the foreach loop with an if condition to catch possible
errors.

Change-Id: I6394ff0eef28a490a54f54d172c601fa9768eb20
Resolves: #73670
Releases: master, 7.6
Reviewed-on: https://review.typo3.org/46925


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Claus Due <claus@phpmind.net>
Reviewed-by: Dmitry Dulepov <dmitry.dulepov@gmail.com>
Tested-by: Dmitry Dulepov <dmitry.dulepov@gmail.com>
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>