[O] small bug in table alignment

[Eric Abrahamsen]

If you call "C-c C-c" within a table to align it, and point happens to
be on a horizontal rule, it throws an error that originates in
org-element-context.

The let at the top of that function sets "type" to 'table-row, and
"element" to:

(table-row (:type rule :begin XX :end XX :contents-begin nil :contents-end nil 
etc...))

Then on line 4749 comes:

  (and (memq type '(paragraph table-row verse-block))
       (let ((cbeg (org-element-property
                    :contents-begin element))
             (cend (org-element-property
                    :contents-end element)))
         (and (>= origin cbeg)
              (<= origin cend)
              (progn (goto-char cbeg) (setq end cend)))))

"cbeg" and "cend" are nil, and strenuously object to being compared
using >= or <=.

I'm not sure what the context of a table rule ought to be, but it will
need to be something with a non-nil :contents-begin value, since that
gets called in `org-ctrl-c-ctrl-c' as well.

Hope that's enough info...

Eric