Issue 51054 - AddressSanitizer: heap-buffer-overflow in ldap_utf8prev
    
    Bug Description:  Adding an invalid/double equal sign when setting the
                      target/targetattr/targetfilter will cause a heap "underflow":
    
                            targetfilter=="(uid=*)"
    
    Fix description:  Detect and reject these invalid ACI syntaxes before we
                      "underflow".  Simply check if the character after the first
                      equal sign is a double quote, as that is the only possible
                      next valid character in a valid ACI.
    
    fixes: https://pagure.io/389-ds-base/issue/51054
    
    Reviewed by: firstyear(Thanks!)