Ticket 49233 - Fix crash in persistent search
    
    Bug Description:  During the fixes to the pblock, we accidentally
    introduced a free to an operation linked to a persistent search.
    
    Fix Description:  NULL the operation in the PB before connection
    threadmain loops.
    
    https://pagure.io/389-ds-base/issue/49233
    
    Author: wibrown
    
    Review by: tbordaz, mreynolds (Thanks!!)
    
    (cherry picked from commit 565314cde101ec1d1d71527ea1b35f12504e9c97)