[Fix #586] Fix emacs freezing when opening file containing "comment"

[OknoLombarda]

Fixes #586
Context #586 (comment)

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[vemv]

lgtm - thanks much for the fix!

[OknoLombarda]

@vemv, @bbatsov, can someone please take a look and accept PR or tell me what else I should improve

[bbatsov]

Btw, is this specific to comment or to just having a single form in the buffer? Looking at the code it's the latter.

[OknoLombarda]

If function clojure-sexp-starts-until-position is called directly, then yes, it will loop forever for any single form in buffer. But this bug is triggered without user doing anything (except for opening the buffer) only when there's just comment in buffer

[vemv]

It would seem wise to create an issue for that edge case - having a more robust building block would make bugs less likely, in a future.

Other than that it seems good to merge this PR in!

[vemv]

Created #652