serial: reject new commands when the port is closed.

Otherwise, we can schedule a main loop call to
mm_serial_port_queue_process() for an object that's about to
disappear, leading to a crash.

Change-Id: I433a76855c52536eb2b99a5ecf26ac71afe1f8bb

src/mm-serial-port.c

@@ -1021,6 +1021,7 @@ internal_queue_command (MMSerialPort *self,
     MMQueueData *info;
 
     g_return_if_fail (MM_IS_SERIAL_PORT (self));
+    g_return_if_fail (priv->open_count > 0);
     g_return_if_fail (command != NULL);
 
     info = g_slice_new0 (MMQueueData);