Do not crash when getting the branches ready for PR on a fork with no parent
    
    If a fork has no parent and you visit it's overview page, this code will
    crash as it will try to access the settings of the parent project that
    no longer exists.
    
    Fixes an error reported automatically by email
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>