netfilter: nf_tables: don't assume chain stats are set when jumplabel is set

nft_chain_stats_replace() and all other spots assume ->stats can be
NULL, but nft_update_chain_stats does not.  It must do this check,
just because the jump label is set doesn't mean all basechains have stats
assigned.

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