Werk #6350: Fixed broken checking of unrelated hosts when using manual checks

When using the Check_MK Raw Edition it could happen that configuring manual checks for a host broke the check execution of unrelated hosts.

The configuration of the manual check of all hosts was loaded into the context of such an unrelated host. Because a host only loads the check definitions of the checks it needs during checking, those manual check configurations were trying to access unavailable data. This lead to a crash and a "(null)" output shown for the "Check_MK" service.

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