[stable9] Fix decrypt message stable9 (#25188)

* Fix Decrypt message via occ

* Comments fixed

* Fixed reviews

* Originally:
commit 2304e4bda027e61ff1302c55c2f70f8e4c8f47d0
Author: Joas Schilling <nickvergessen@owncloud.com>
Date:   Tue Jun 7 09:13:11 2016 +0200

    Allow to decrypt user '0' files only

* Fix uid comparison