CVE-2015-5330: ldb_dn_explode: copy strings by length, not terminators

That is, memdup(), not strdup(). The terminators might not be there.

But, we have to make sure we put the terminator on, because we tend to
assume the terminator is there in other places.

Use talloc_set_name_const() on the resulting chunk so talloc_report()
remains unchanged.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11599

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Pair-programmed-with: Andrew Bartlett <abartlet@samba.org>
Pair-programmed-with: Garming Sam <garming@catalyst.net.nz>
Pair-programmed-with: Stefan Metzmacher <metze@samba.org>
Pair-programmed-with: Ralph Boehme <slow@samba.org>
