segtree: fix map listing with interface wildcard

segtree: fix map listing with interface wildcard

 # nft -f - <<'EOF'
 table inet filter {
    chain INPUT {
        iifname vmap {
            "eth0" : jump input_lan,
            "wg*" : jump input_vpn
        }
    }
    chain input_lan {}
    chain input_vpn {}
 }
 EOF
 # nft list ruleset
 nft: segtree.c:578: interval_map_decompose: Assertion `low->len / 8 > 0' failed.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1617
Fixes: 5e393ea1fc0a ("segtree: add string "range" reversal support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>