Fix multi-serial CRC error crash (#21249)

Marlin/src/gcode/control/M999.cpp

 
   if (parser.boolval('S')) return;
 
-  queue.flush_and_request_resend();
+  queue.flush_and_request_resend(queue.ring_buffer.command_port());
 }

Marlin/src/gcode/queue.cpp

  * Send a "Resend: nnn" message to the host to
  * indicate that a command needs to be re-sent.
  */
-void GCodeQueue::flush_and_request_resend() {
-  const serial_index_t serial_ind = ring_buffer.command_port();
+void GCodeQueue::flush_and_request_resend(const serial_index_t serial_ind) {
   #if HAS_MULTI_SERIAL
     if (serial_ind < 0) return;                   // Never mind. Command came from SD or Flash Drive
     PORT_REDIRECT(SERIAL_PORTMASK(serial_ind));   // Reply to the serial port that sent the command
   SERIAL_ERROR_START();
   SERIAL_ECHOLNPAIR_P(err, serial_state[serial_ind].last_N);
   while (read_serial(serial_ind) != -1) { /* nada */ } // Clear out the RX buffer. Why don't use flush here ?
-  flush_and_request_resend();
+  flush_and_request_resend(serial_ind);
   serial_state[serial_ind].count = 0;
 }
 

Marlin/src/gcode/queue.h

    * Clear the serial line and request a resend of
    * the next expected line number.
    */
-  static void flush_and_request_resend();
+  static void flush_and_request_resend(const serial_index_t serial_ind);
 
   /**
    * (Re)Set the current line number for the last received command