Valdis reports NULL deref in nf_ct_frag6_gather.
Problem is bogus use of skb_queue_walk() -- we miss first skb in the list
since we start with head->next instead of head.
In case the element we're looking for was head->next we won't find
a result and then trip over NULL iter.
(defrag uses plain NULL-terminated list rather than one terminated by
head-of-list-pointer, which is what skb_queue_walk expects).