common: don't call _find_linked_issues_harder if no fix_versions

It's possible for find_linked_issues to get called for an issue with no fix_versions, and that was causing _find_linked_issues_harder to blow up. Fix that, but also, make sure if we get a list of issues, and some don't have a fix_version set, we skip trying to find matches for those ones, but still return results for the others.

Signed-off-by: Jarod Wilson jarod@redhat.com