Ticket #47313 - Indexed search with filter containing '&' and "!" with attribute subtypes gives wrong result
    
    Description: commit fae006821bd6e524c0f7f8d5f023f4fe5e160ef0 introduced
    a bug, which occurs when a filter includes NOT and one of the results
    from the subfilters returns NONE. This patch backoffs the last section
    of the commit fae006821bd6e524c0f7f8d5f023f4fe5e160ef0 with an improvement
    -- avoiding unnecessary idl duplication.
    
    Also, adding (NULL == idl) checks to idl_common.c.
    
    https://fedorahosted.org/389/ticket/47313
    
    Reviewed by rmeggins@redhat.com (Thank you, Rich!)
    (cherry picked from commit a71633d56951dd6c4d0368c790b85628f1598968)