Skip N/A sensor values

By default, sensor values that don't return valid values are still considered. This causes errors on the following arithmetic calculations, so this commit fixes it.

Compare this output:
    
    sensors |grep -E "^(Core|temp)"     
    temp1:        +44.0°C  (crit = +127.0°C)  
    temp2:        +34.0°C  (crit = +100.0°C)
    Core 0:       +35.0°C  (high = +105.0°C, crit = +105.0°C)
    Core 1:       +34.0°C  (high = +105.0°C, crit = +105.0°C)
    temp1:        +44.0°C  
    temp2:            N/A  
    temp3:            N/A  
    temp4:            N/A  
    temp5:        +30.0°C  
    temp6:            N/A  
    temp7:        +28.0°C  
    temp8:            N/A  
    temp9:        +28.0°C  
    temp10:       +36.0°C  
    temp11:       +37.0°C  
    temp12:           N/A  
    temp13:           N/A  
    temp14:           N/A  
    temp15:           N/A  
    temp16:           N/A  

This pull request changes this to:

    temp1:        +45.0°C  (crit = +127.0°C)
    temp2:        +36.0°C  (crit = +100.0°C)
    Core 0:       +35.0°C  (high = +105.0°C, crit = +105.0°C)
    Core 1:       +37.0°C  (high = +105.0°C, crit = +105.0°C)
    temp1:        +45.0°C  
    temp5:        +30.0°C  
    temp7:        +28.0°C  
    temp9:        +29.0°C  
    temp10:       +36.0°C   
    temp11:       +37.0°C