Ticket 50300 - Fix memory leak in automember plugin
    
    Description:  We were allocating a pblock long before it was used, and
                  we were returning from the function on an error before we
                  freed it.  The fix just allocates the pblock right before
                  it's used, and then it is properly freed.
    
    https://pagure.io/389-ds-base/issue/50300
    
    Reviewed by: mreynolds (one line commit rule)