LDAP: Systemtap ldap probes fail without filter
    
    SSSD systemtap probes for LDAP queries doesn't check the ldap search
    filter. It's value can be NULL (means "no filter") and stap fails with
    
        ERROR: user string copy fault -14 at ...
    
    This patch tests filter value before it is converted to printable
    string and the filter is set to "<no filter>" in case of NULL value.
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>