Add 1 to available() if peek has been previously called

Fixes #405; a proper fix require to use another RingBuffer (useless since the USB data is already internally buffered)

Author: facchinm

cores/arduino/USB/CDC.cpp

@@ -199,9 +199,11 @@ void Serial_::end(void)
 	memset((void*)&_usbLineInfo, 0, sizeof(_usbLineInfo));
 }
 
+int _serialPeek = -1;
+
 int Serial_::available(void)
 {
-	return usb.available(CDC_ENDPOINT_OUT);
+	return usb.available(CDC_ENDPOINT_OUT) + (_serialPeek != -1);
 }
 
 int Serial_::availableForWrite(void)
@@ -211,8 +213,6 @@ int Serial_::availableForWrite(void)
 	return (EPX_SIZE - 1);
 }
 
-int _serialPeek = -1;
-
 int Serial_::peek(void)
 {
 	if (_serialPeek != -1)