Unable to read signatures in comments whose input format is not accessible to the current user

I have a "Filtered HTML Extended" input format which is used by a few people (editors and moderators) on a site. Normal members and anonymous users don't have access to this input format.

The problem begins when a comment is submitted in the above input format ("Filtered HTML Extended"). When a user that doesn't have access to this input format views that comment, the signature will read "n/a".

I've tracked this down to check_markup(), which user.module calls in user_comment(), to filter the signature before appending it to the comment. I believe FALSE should be passed as a third parameter to check_markup(), which effectively bypasses the filter_access() check, allowing anyone to view the signature without requiring them to have access to the input format the comment is written in.

As I'm not an experienced Drupal developer, I may be missing something critical here, so I'm attaching my proposed patch for review.

Any feedback is welcome! :)