send nak symbol in case of faulty header

src/shellmatta_ymodem.c

@@ -487,7 +487,7 @@ void shellmatta_ymodem_nak(shellmatta_handle_t handle)
         break;
     
     case YMODEM_HEADER:
-        // shellmatta_ymodem_current_data_type = YMODEM_NONE;
+        shellmatta_ymodem_control(handle, YMODEM_NAK);
         break;
 
     case YMODEM_BODY: