Update HardwareSerial.cpp

If called second time - fix the counts to reflect just the size of the new buffer plus the static buffer.

Also as a precaution, resets the TX or RX head tail pointers back to 0 to make sure it is not indexing into memory that may not exist.

teensy4/HardwareSerial.cpp

 {
 	rx_buffer_storage_ = (BUFTYPE*)buffer;
 	if (buffer) {
-		rx_buffer_total_size_ = rx_buffer_total_size_ + length;
+		rx_buffer_total_size_ = rx_buffer_size_ + length;
 	} else {
-		rx_buffer_total_size_ = rx_buffer_total_size_;
+		rx_buffer_total_size_ = rx_buffer_size_;
 	} 
 
+	// Make sure we don't end up indexing into no mans land. 
+	rx_buffer_head_ = 0;
+	rx_buffer_tail_ = 0;
 	rts_low_watermark_ = rx_buffer_total_size_ - hardware->rts_low_watermark;
 	rts_high_watermark_ = rx_buffer_total_size_ - hardware->rts_high_watermark;
 }
 {
 	tx_buffer_storage_ = (BUFTYPE*)buffer;
 	if (buffer) {
-		tx_buffer_total_size_ = tx_buffer_total_size_ + length;
+		tx_buffer_total_size_ = tx_buffer_size_ + length;
 	} else {
-		tx_buffer_total_size_ = tx_buffer_total_size_;
+		tx_buffer_total_size_ = tx_buffer_size_;
 	} 
+	// Make sure we don't end up indexing into no mans land. 
+	tx_buffer_head_ = 0;
+	tx_buffer_tail_ = 0;
 }
 
 int HardwareSerial::peek(void)