The getTempCByIndex function doesn't check getAddress return value

[JoakimSoderberg]

When calling getTempCByIndex and no sensors are available on the 1-wire network (deviceCount == 0), the call to getAddress will return false. But in the code this is never checked for.

It should return DEVICE_DISCONNECTED_C message, but now I'm simply getting 0.0

https://github.com/milesburton/Arduino-Temperature-Control-Library/blob/master/DallasTemperature.cpp#L403

[milesburton]

Many thanks, merged