Werk #2959: apt: Made check more robust against invalid agent data

Component	Checks & agents
Title	apt: Made check more robust against invalid agent data
Date	Jan 26, 2016
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.2.7i4 Checkmk Community, Checkmk Pro, Checkmk Ultimate MT

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