Fix assertion in change tracking.

Happens when there are changes to a disabled product's source artifacts.
This was introduced by change 78abe57794, which removed one line too
many.

Change-Id: If9b6ce90282eaf13670c0c6af8db42b642b9733b
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>