Werk #1791: Fix problem where many bogus RRD files for Check_MK service would be created

The problem appeared when using CMC and during check execution some general problem appeared like UNKNOWN - global name 'netsnmp' is not defined. Then in the next check done by the same helper this message would appear again because one variable was not cleared. That garbled the protocol with the microcore and the later took the output of the passive checks as performance data. This has been fixed.

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