netlink: fix element addition to map with stateful object

netlink: fix element addition to map with stateful object

Expressions with EXPR_F_INTERVAL_END flag set on have no right hand side,
so they store no stateful object. Skip them so we don't crash on this.

 # nft add map x testmap { type inet_service: counter\; flags interval\;}
 # nft add counter x testcounter
 # nft add element x testmap { 0-100 : "testcounter" }
 Segmentation fault

This patch also fixes the listing codepath.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1190
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>