iCalendar export: Fix problem with match data.

iCalendar export: Fix problem with match data.

Harri Kiiskinen writes:

> There is an elusive bug in the ICal export functions. To
> demonstrate it, open this file, allow for the use of the local
> variable, and run 'org-export-icalendar-this-file'. What I get is
> the message "Wrong type argument: stringp, nil"
>
> -----------------------------
> #+FILETAGS: test
> * TODO header
>
> # Local Variables:
> # org-use-tag-inheritance: "^[t]"
> # End:
> -----------------------------
>
> Now, the funny thing is, that if you change the filetag from
> "test" to "est", close the file, and open it again, the file will
> be exported fine! It seems, that if the filetag does match the
> regular expression in org-use-tag-inheritance, then the result is
> error.

This bug was due to a missing save-match-data around a call to
org-get-tags-at.  However, when studying this bug it turned out
that explicitly checking for the archive tag is not needed,
because we already have a call to `org-agenda-skip' in the same
loop.