Arduino hangs when reading from non-existent pins.

[AwesomeTornado]

the hang happens in the function pinPeripheral in the file wiring_private.c at line 96 or 105 (it changes depending on an if statement) when the function tries to multiplex the pin number.

this can be easily fixed by adding the code below to the analogRead function at line 135 in the file wiring_analog.c

  if (g_APinDescription[pin].ulPinType != PIO_ANALOG){
    return  -1;
  }

this will check if the pin is an analog pin before trying to read/multiplex it.

you can read more on the Arduino forums here.