Fixed regression in USBAPI and CDC (xaljox)

See:
arduino/Arduino@9ac7e30#commitcomment-6718676

Author: cmaglie

arduino/cores/arduino/CDC.cpp

@@ -116,10 +116,12 @@ bool WEAK CDC_Setup(Setup& setup)
 
 void Serial_::begin(unsigned long baud_count)
 {
+	peek_buffer = -1;
 }
 
 void Serial_::begin(unsigned long baud_count, byte config)
 {
+	peek_buffer = -1;
 }
 
 void Serial_::end(void)

arduino/cores/arduino/USBAPI.h

@@ -28,8 +28,9 @@ extern USBDevice_ USBDevice;
 class Serial_ : public Stream
 {
 private:
-	int peek_buffer = -1;
+	int peek_buffer;
 public:
+	Serial_() { peek_buffer = -1; };
 	void begin(unsigned long);
 	void begin(unsigned long, uint8_t);
 	void end(void);