netfilter: xtables: avoid BUG_ON

I see no reason for them, label or timer cannot be NULL, and if they
were, we'll crash with null deref anyway.

For skb_header_pointer failure, just set hotdrop to true and toss
such packet.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>