Fix bug with protected examples that look like table lines

Fix bug with protected examples that look like table lines

* lisp/org-exp.el (org-export-remove-special-table-lines): Only fix table
lines that are not protected text.

Giovanni Moretti writes:

> I'm working up a presentation on orgmode for a local club and needed
> to prefix it with a brief emacs overview, and so included this:
>
> #+BEGIN_EXAMPLE
>                      ctrl-P (previous line)
>                           |
>                           |
> Ctrl-A <<<  Ctrl-B <---- o ----> Ctrl-F >>>  Ctrl-E
> Col 1      back char      !       fwd char        EOL
>                           !
>                      ctrl-N (next line)
> #+END_EXAMPLE
>
> :                       ctrl-P (previous line)
> :                            |
> :                            |
> : Ctrl-A <<<  Ctrl-B <---- o ----> Ctrl-F >>>  Ctrl-E
> : Col 1       back char      |       fwd char        EOL
> :                            |
> :                       ctrl-N (next line)
>
> I'm using Orgmode v6.36c and when exporting to HTML (and LaTex
> Beamer), the two lines containing the single vertical bar immediately
> below the "ctrl-P" line in the #+EXAMPLE block vanish, whereas using
> the alternate colon at the beginning of the line notation, the
> rendering is as expected.
>
> Interestingly, enabling the +n option (#+BEGIN_EXAMPLE +n) causes the
> missing lines (lines 2 & 3) to reappear.