Merge pull request #1870 from matthijskooijman/ide-1.5.x-serial-int

In HardwareSerial::_rx_complete_irq, don't use int for buffer index

Author: cmaglie

cores/arduino/HardwareSerial_private.h

@@ -99,7 +99,7 @@ void HardwareSerial::_rx_complete_irq(void)
     // No Parity error, read byte and store it in the buffer if there is
     // room
     unsigned char c = *_udr;
-    int i = (unsigned int)(_rx_buffer_head + 1) % SERIAL_BUFFER_SIZE;
+    uint8_t i = (unsigned int)(_rx_buffer_head + 1) % SERIAL_BUFFER_SIZE;
 
     // if we should be storing the received character into the location
     // just before the tail (meaning that the head would advance to the