Possible division by zero error in NTCSensor

sensors/libraries/NTCSensor/NTCSensor.cpp

Lines 196 to 197 in 9c22916

    
           average = 1023.0 / average - 1; 
        
           average = SERIES_RESISTOR / average;

Having shorted thermistor is handled but having not a thermistor at all results in pulling analog in to VCC and counted as 1023. This in turn will result in division by zero in line 197 in NTCSensor.cpp.

Fixed in #6

	average = 1023.0 / average - 1;
	average = SERIES_RESISTOR / average;