expression: tolerate named set protocol dependency

expression: tolerate named set protocol dependency

Included test will fail with:
/dev/stdin:8:38-52: Error: Transparent proxy support requires transport protocol match
   meta l4proto @protos tproxy to :1088
                        ^^^^^^^^^^^^^^^
Tolerate a set reference too.  Because the set can be empty (or there
can be removals later), add a fake 0-rhs value.

This will make pctx_update assign proto_unknown as the transport protocol
in use, Thats enough to avoid 'requires transport protocol' error.

v2: restrict it to meta lhs for now (Pablo Neira Ayuso)

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1686
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>