dns scavenging: ensure usual ownership of element values

dns scavenging: ensure usual ownership of element values

An ldb message, its elements, and their values usually all share a
little talloc sub-tree with each other and nobody else. It is
conceivable that somewhere we rely on that.

In this case we were sharing an out-of-subtree values array across
multiple messages, which seems to be asking for trouble.

Also, add a comment explaining what we want.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>