VPP-1337: Dangling reference be gone!

Freeing the prefix and address vectors after signalling them from
dhcpv6_client_node_fn to either the pd or iana report processes is
absolutely wrong. The code has appeared to work by luck.

This took three full days' worth of expert effort to root cause - in
the context of an unrelated patch - and about one minute to fix.

Change-Id: Ie589ab1a70ae83441ab4e8b6618a6d405741f53f
Signed-off-by: Dave Barach <dave@barachs.net>