s4:bind_dlz: Only copy the dn once

s4:bind_dlz: Only copy the dn once

../source4/dns_server/dlz_bind9.c:1132:4: error: 'dn' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    state->log(ISC_LOG_INFO, "failed to find dnsRecord for %s",
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        ldb_dn_get_linearized(dn));
        ~~~~~~~~~~~~~~~~~~~~~~~~~~

Found by Undefined Sanitizer.

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>