Check_MK Werk 3918: Inventory history: Fixed possible exception when renedering history tables

ID: 3918 Title: Inventory history: Fixed possible exception when renedering history tables Component: HW/SW-Inventory Level: 1 Class: Bug Fix Version: 1.4.0i3 Different kind of exceptions could occur, all in the inv_*() functions. For example in inv_paint_timestamp_as_age() the error "TypeError: unsupported operand type(s) for -: 'float' and 'tuple'"