atsam: add a length check to findPinPadMapping

src/machine/machine_atsamd21.go

@@ -136,6 +136,11 @@ var pinPadMapping = [32]byte{
 // found" (indicated by returning ok=false). The pad number is returned to
 // calculate the DOPO/DIPO bitfields of the various serial peripherals.
 func findPinPadMapping(sercom uint8, pin Pin) (pinMode PinMode, pad uint32, ok bool) {
+	if int(pin)/2 >= len(pinPadMapping) {
+		// This is probably NoPin, for which no mapping is available.
+		return
+	}
+
 	nibbles := pinPadMapping[pin/2]
 	upper := nibbles >> 4
 	lower := nibbles & 0xf