[NETFILTER]: Fix oops in conntrack event cache

ip_ct_refresh_acct() can be called without a valid "skb" pointer.
This used to work, since ct_add_counters() deals with that fact.
However, the recently-added event cache doesn't handle this at all.

This patch is a quick fix that is supposed to be replaced soon by a cleaner
solution during the pending redesign of the event cache.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>