1) If a spider crawled on your face, you will most likely feel it, so a spider would have to enter your mouth by "abseiling" into it from the ceiling on its silk thread;
2) Then the spider would have to hit your mouth dead centre to avoid tickling your lips and arousing your attention;
3) Even if it did that the chances are it would then hit your tongue, which has a highly-sensitive surface. You would almost certainly feel it. This would mean that the spider would have to land at the back of your throat without touching anything on the way in;
4) And, after all that, you would then have to swallow at just the right moment.
So it is very silly to worry about swallowing spiders in your sleep because, basically, it is not going to happen.