segtree: add missing non-matching segment to set in flat representation

segtree: add missing non-matching segment to set in flat representation

 # cat test.nft
 add set x y { type ipv4_addr; }
 add element x y { 10.0.24.0/24 }
 # nft -f test.nft
 # nft delete element x y { 10.0.24.0/24 }

bogusly returns -ENOENT. The non-matching segment (0.0.0.0 with end-flag
set on) is not added to the set in the example above.

This patch also adds a test to cover this case.

Fixes: 4935a0d561b5 ("segtree: special handling for the first non-matching segment")
Reported-by: Václav Zindulka <vaclav.zindulka@tlapnet.cz>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>