CVE-2016-0771: tests/dns: restore formerly segfaulting test

This was on the client side, due the a strlen(NULL) on the previously
DOS-encoded TXT field. With a new IDL structure, this segfault no longer exists.
Note that both Samba and Windows return NXRRSET instead of FORMERR.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11128
BUG: https://bugzilla.samba.org/show_bug.cgi?id=11686

Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
