netfilter: nft_ct: fix random validation errors for zone set support

Dan reports:
 net/netfilter/nft_ct.c:549 nft_ct_set_init()
 error: uninitialized symbol 'len'.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: edee4f1e ("netfilter: nft_ct: add zone id set support")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>