optimize: expand expression list when merging into concatenation

optimize: expand expression list when merging into concatenation

The following rules:

    udp dport 137 ct state new,untracked accept
    udp dport 138 ct state new,untracked accept

results in:

  nft: src/optimize.c:670: __merge_concat: Assertion `0' failed.

The logic to expand to the new,untracked list in the concatenation is
missing.

Fixes: 187c6d01d357 ("optimize: expand implicit set element when merging into concatenation")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>