Bug 36791: Koha explodes when trying to edit an authority rec. with an invalid authid

Bug 36791: Koha explodes when trying to edit an authority rec. with an invalid authid

When trying to open the authority editor with authid=<invalid_authid>
(e.g. a deleted authid) Koha explodes with a message:
Can't call method "authtypecode" on an undefined value at
/kohadevbox/koha/authorities/authorities.pl line 556

This this because authtypecode method is called on the result of
->find without verifying that it was succesful.

Test plan:
==========
1. Try to edit an auth rec. giving as a authid (in URL) a non-existing
   authid, e.g. in ktd, with standard ktd test data:
   http://your_ktd:8081/cgi-bin/koha/authorities/authorities.pl?authid=100000
   Koha should explode with the message:
   Can't call method "authtypecode" on an undefined value at
   /kohadevbox/koha/authorities/authorities.pl line 556
2. Apply the patch; restart_all.
3. Repeat p. 1.  You should get the 404 error page.

Sponsored-by: Ignatianum University in Cracow
Signed-off-by: Roman Dolny <roman.dolny@jezuici.pl>
Signed-off-by: Emily Lamancusa <emily.lamancusa@montgomerycountymd.gov>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>