fix index corruption race condition

lib/usart.c

@@ -105,9 +105,11 @@ char usart_getchar(void)
 		__WFI(); // sleep until interrupt;
 	}
 	char to_return = rx_buffer[rx_i]; // get the next available character
+	usart_disable_rx_interrupt(USART); // disable receive interrupt to prevent index corruption
 	rx_i = (rx_i+1)%sizeof(rx_buffer); // update used buffer
 	rx_used--; // update used buffer
 	usart_received = rx_used; // update available data
+	usart_enable_rx_interrupt(USART); // enable receive interrupt
 	return to_return;
 }
 
@@ -117,6 +119,7 @@ void usart_putchar_nonblocking(char c)
 		usart_enable_tx_interrupt(USART); // enable transmit interrupt
 		__WFI(); // sleep until something happened
 	}
+	usart_disable_tx_interrupt(USART); // disable transmit interrupt to prevent index corruption
 	tx_buffer[(tx_i+tx_used)%sizeof(tx_buffer)] = c; // put character in buffer
 	tx_used++; // update used buffer
 	usart_enable_tx_interrupt(USART); // enable transmit interrupt