Ticket 47858 - Internal searches using OP_FLAG_REVERSE_CANDIDATE_ORDER can crash the server
    
    Bug Description:  If an internal search uses OP_FLAG_REVERSE_CANDIDATE_ORDER, and
                      the search fails tro find any candidates the server will crash.
    
    Fix Description:  Make sure we do not dereference a NULL sr_candidates pointer in
                      ldbm_search().
    
    https://fedorahosted.org/389/ticket/47858
    
    Reviewed by: rmeggins(Thanks!)
    
    (cherry picked from commit e6cee31aa2beb6496df86490776f1f93d3a8348b)
    (cherry picked from commit da318fa5f147e229069b13c0479fdf81ccc28213)