Print an error when serial number is invalid (#691)

* Print error when serial number is invalid

* Move to the first check, change end by return

cores/esp32/HardwareSerial.cpp

@@ -11,6 +11,10 @@ HardwareSerial::HardwareSerial(int uart_nr) : _uart_nr(uart_nr), _uart(NULL) {}
 
 void HardwareSerial::begin(unsigned long baud, uint32_t config, int8_t rxPin, int8_t txPin)
 {
+    if(0 > _uart_nr || _uart_nr > 2) {
+        log_e("Serial number is invalid, please use 0, 1 or 2");
+        return;
+    }
     if(_uart) {
         end();
     }