"Similar Tags" module ignores access permissions for tagged articles

Bug reported by Thomas Schmitz:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33822&start=0

The fix also checks the access level of the corresponding articles.

How to reproduce bug (from JoomlaCode entry):

1) Create a tag 
2) Assign the tag to an article with "Public" access and another article with "Registered" access 
3) Now an unregistered user will see a link to both articles with the "Similar Tag" module