Werk #10621: Fix bug where scheduler would become inactive

Component	Core & setup
Title	Fix bug where scheduler would become inactive
Date	Feb 19, 2020
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.0.0i1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.6.0p10 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.5.0p25 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

Certain configurations let CMC schedule jobs with invalid timestamps. These jobs would never be scheduled and let the schedulers in a nonoperating state, resulting in stale hosts and services.

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