cljr-slash: rule out some edge cases

[vemv]

For cljr-slash functionality, some edge cases will predictably never succeed, yet will spend computational time.

e.g. a/a/ has two slashes, there will be nothing matching that.

Similarly sometimes as I use emacs I make other typos that also can never succeed, yet block the UI for a few secs.

So one could have a simple regex for validating the expected admissible inputs.