[BUGFIX] Keep scheduler functional after task interruption

If the execution of a task was interrupted (e.g., for debugging
purpose with Xdebug), the scheduler module may not open up again
and fails with a PHP warning.

Change-Id: I8d28d2094eeff941b2c09788480e8b79bef766f1
Releases: master, 6.2
Resolves: #66537
Reviewed-on: http://review.typo3.org/38856


Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Andreas Fernandez <andreas.fernandez@aspedia.de>
Tested-by: Andreas Fernandez <andreas.fernandez@aspedia.de>