Ticket 47525 - Crash if setting invalid plugin config area for MemberOf Plugin
    
    Bug Description:  Setting the nsslapd-pluginconfigarea to an entry that
                      does not have the required config attributes causes a
                      crash.
    
    Fix Description:  The plugin entry was being accidentally freed instead
                      of the config area entry.
    
                      The shared config area validation was being performed
                      in postop - this has now been moved into the preop stage.
    
                      Also, set the returntext when an error occurs.
    
    https://fedorahosted.org/389/ticket/47525
    
    Reviewed by: rmeggins(Thanks!)
    
    (cherry picked from commit 42f935ab7406802d522f357048db1e68d729d5e5)