Update PerfdataValue::Parse to ignore invalid warn, crit, min, and max values

Update PerfdataValue::Parse to ignore invalid warn, crit, min, and max values

... instead of generating exception so that metric values can
still be passed to the graphing backend.
Also update icinga-perfdata tests to reflect these changes.

refs #5043

Signed-off-by: Michael Friedrich <michael.friedrich@netways.de>