Check if the process-mark is non-nil and if mark-pos is non-nil to prevent errors

First, wanted to say that I love crdt.el. Been super useful for pairing. I found when trying to share cider-repls with it, I ran into mark being nil. Maybe I was a little too cautious here, but I went ahead and checked for both mark and mark-pos to make sure it wouldn't error out if either was nil.

If you want any changes let me know.