netlink_delinarize: don't check for set element if set is not populated

netlink_delinarize: don't check for set element if set is not populated

0065_icmp_postprocessing: line 13: Segmentation fault $NFT insert rule ip x foo index 1 accept

Since no listing is done, cache isn't populated and 'nft insert' will trip over
set->init == NULL during postprocessing of the existing 'icmp id 42' expression.

Fixes: 9a5574e2d4e9 ("netlink_delinearize: add missing icmp id/sequence support")
Reported-by: Eric Garver <eric@garver.life>
Reported-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>