Disable the UDRE interrupt sooner in HardwareSerial

Before, the interrupt was disabled when it was triggered and it turned
out there was no data to send. However, the interrupt can be disabled
already when the last byte is written to the UART, since write() will
always re-enable the interrupt when it adds new data to the buffer.

Closes: arduino#1008

Author: matthijskooijman

Diff for: hardware/arduino/cores/arduino/HardwareSerial.cpp

@@ -220,22 +220,22 @@ void HardwareSerial::_rx_complete_irq(void)
 
 void HardwareSerial::_tx_udr_empty_irq(void)
 {
+  // If interrupts are enabled, there must be more data in the output
+  // buffer. Send the next byte
+  unsigned char c = _tx_buffer[_tx_buffer_tail];
+  _tx_buffer_tail = (_tx_buffer_tail + 1) % SERIAL_BUFFER_SIZE;
+
+  *_udr = c;
+
+  // clear the TXC bit -- "can be cleared by writing a one to its bit
+  // location". This makes sure flush() won't return until the bytes
+  // actually got written
+  sbi(*_ucsra, TXC0);
+
   if (_tx_buffer_head == _tx_buffer_tail) {
     // Buffer empty, so disable interrupts
     cbi(*_ucsrb, UDRIE0);
   }
-  else {
-    // There is more data in the output buffer. Send the next byte
-    unsigned char c = _tx_buffer[_tx_buffer_tail];
-    _tx_buffer_tail = (_tx_buffer_tail + 1) % SERIAL_BUFFER_SIZE;
-
-    *_udr = c;
-
-    // clear the TXC bit -- "can be cleared by writing a one to its bit
-    // location". This makes sure flush() won't return until the bytes
-    // actually got written
-    sbi(*_ucsra, TXC0);
-  }
 }
 
 // Constructors ////////////////////////////////////////////////////////////////