Fix a bug in calculation of nextRun for pcron

taskman-ng was scanning only the todo list, not the full list of all tasks and thus
missing tasks that should run sooner. Happily, this was fixed by
removing code and adding one additional check in runTask

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Review fix: missing cbLoop if no config returned
Merge pull request #764 from Singly/dss-fix-pcron-nextrun-calc

Fix a bug in calculation of nextRun for pcron