Let SoftwareSerial::end also check against _rx_delay_stopbit

The current check is still always false when the old check was, but
additionally it will not disable the interrupts when they were never
enabled (which shouldn't matter much, but this is more consistent).

libraries/SoftwareSerial/SoftwareSerial.cpp

@@ -418,7 +418,7 @@ void SoftwareSerial::begin(long speed)
 
 void SoftwareSerial::end()
 {
-  if (digitalPinToPCMSK(_receivePin))
+  if (_rx_delay_stopbit)
     *digitalPinToPCMSK(_receivePin) &= ~_BV(digitalPinToPCMSKbit(_receivePin));
 }