Prevent buffer retransmission when transmit buffer is empty

Moving the head buffer pointer and setting interrupt flag is now
atomic in write(). Previously an intervening ISR could empty the
buffer before the second ISR is triggered causing retransmission.

Fixes: #3745 (original issue only)

cores/arduino/HardwareSerial.cpp

@@ -246,9 +246,14 @@ size_t HardwareSerial::write(uint8_t c)
   }
 
   _tx_buffer[_tx_buffer_head] = c;
-  _tx_buffer_head = i;
-	
-  sbi(*_ucsrb, UDRIE0);
+
+  // make atomic to prevent execution of ISR between setting the
+  // head pointer and setting the interrupt flag resulting in buffer
+  // retransmission
+  ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+    _tx_buffer_head = i;
+    sbi(*_ucsrb, UDRIE0);
+  }
   
   return 1;
 }