Check_MK Werk 6220: df: Fix crashing check for vanished filesystems

ID: 6220 Title: df: Fix crashing check for vanished filesystems Component: Checks & agents Level: 1 Class: Bug fix Version: 1.6.0i1 Up to and including 1.4.0*, df would report UNKNOWN when confronted with a vanished filesystem. A regression in 1.5.0 versions previous to this lead to the check crashing instead. This has been fixed.