[NETFILTER] ctnetlink: Fix oops when no ICMP ID info in message

This patch fixes an userspace triggered oops. If there is no ICMP_ID
info the reference to attr will be NULL.

Signed-off-by: Krzysztof Piotr Oledzki <ole@ans.pl>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>