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.

merged

mentioned in commit 81b55a27

Thanks for the patch!

I'm curious if cider-repl with crdt.el works for you? If it is not comint-based or a heavily modified comint (which is likely to be the case), I think it will likely not work without some proper annotations from the package side.

I'm mostly working on 0.3.0 recently and hopefully we'll have a stable API for package authors by then, so people can start integrating different packages.