iptables: fix memory corruption when parsing nft rules

iptables: fix memory corruption when parsing nft rules

This commit fixes a memory corruption issue when iptables (with
enabled PACKAGECONFIG libnftnl) is used to access rules created by
nft.

To reproduce the issue:
nft add chain ip filter TESTCHAIN { meta mark set 123 \;}
iptables -t filter -n -L TESTCHAIN

This produced the following output:
Chain TESTCHAIN (0 references)
target prot opt source     destination
MARK   0    --  0.0.0.0/0  0.0.0.0/0     MARK set 0x7b
malloc(): corrupted top size
Aborted (core dumped)

This commit fixes this issue.

Signed-off-by: Christian Taedcke <christian.taedcke@weidmueller.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>