netfilter: nft_set_pipapo: skip inactive elements during set walk

Otherwise set elements can be deactivated twice which will cause a crash.

Reported-by: Xingyuan Mo <hdthky0@gmail.com>
Fixes: 3c4287f6

 ("nf_tables: Add set type for arbitrary concatenation of ranges")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>