netfilter: ctnetlink: fix refcnt leak in dying/unconfirmed list dumper

'last' keeps track of the ct that had its refcnt bumped during previous
dump cycle.  Thus it must not be overwritten until end-of-function.

Another (unrelated, theoretical) issue: Don't attempt to bump refcnt of a conntrack
whose reference count is already 0.  Such conntrack is being destroyed
right now, its memory is freed once we release the percpu dying spinlock.

Fixes: b7779d06 ('netfilter: conntrack: spinlock per cpu to protect special lists.')
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>