iptables-compat: fix empty chains after first invocation of iptables-compat -L

iptables-compat: fix empty chains after first invocation of iptables-compat -L

 # iptables-compat -L
 # iptables-compat -L
 Chain INPUT (policy ACCEPT)
 target     prot opt source               destination

 Chain FORWARD (policy ACCEPT)
 target     prot opt source               destination

 Chain OUTPUT (policy ACCEPT)
 target     prot opt source               destination

Note that the second (and follow up) invocations after the first one
display the chains.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>