netfilter: ipset: timeout fixing bug broke SET target special timeout value

The patch "127f5591 netfilter: ipset: fix timeout value overflow bug"
broke the SET target when no timeout was specified.

Reported-by: Jean-Philippe Menil <jean-philippe.menil@univ-nantes.fr>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>