Handle missing MinValue/MaxValue

In the existing implementation, if MinValue/MaxValue
property is missing on dbus and if one of the
warning or critical thresholds are set, then the
readings are scaled differently in `ipmiSenGetSensorReading`
than the values reported for sensor attributes(B, M, bExp, rExp)
in `ipmiStorageGetSDR`.

Signed-off-by: Shakeeb Pasha <shakeebbk@gmail.com>
Change-Id: I9cd10224985874d8e200b6dcfaf0e9394575d9d9
1 file changed