Fix use SW-Serial if defiend

Author: NicolasBockstaller-EH

Adafruit_Fingerprint.cpp

@@ -144,7 +144,12 @@ void Adafruit_Fingerprint::begin(uint32_t baudrate, int8_t rxPin,
                                  int8_t txPin) {
   delay(1000); // one second delay to let the sensor 'boot up'
 
-  hwSerial->begin(baudrate, SERIAL_8N1, rxPin, txPin);
+  if (hwSerial)
+    hwSerial->begin(baudrate, SERIAL_8N1, rxPin, txPin);
+  else if (swSerial)
+    swSerial->begin(baudrate, SERIAL_8N1, rxPin, txPin);
+  else
+    throw "No serial-interface defined";
 }
 #endif