netfilter: ipset: dumping error triggered removing references twice

If there was a dumping error in the middle, the set-specific variable was
not zeroed out and thus the 'done' function of the dumping wrongly tried
to release the already released reference of the set. The already released
reference was caught by __ip_set_put and triggered a kernel BUG message.
Reported by Jean-Philippe Menil.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>