Werk #6406: df: fix a crash of the check if the inode count is '-'

Component	Checks & agents
Title	df: fix a crash of the check if the inode count is '-'
Date	Aug 31, 2018
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.6.0b1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.5.0p3 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

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