evaluate: don't allow nat map with specified protocol

evaluate: don't allow nat map with specified protocol

Included bogon asserts:
src/netlink_linearize.c:1305: netlink_gen_nat_stmt: Assertion `stmt->nat.proto == NULL' failed.

The comment right above the assertion says:
  nat_stmt evaluation step doesn't allow
  STMT_NAT_F_CONCAT && stmt->nat.proto.

... except it does allow it.  Disable this.

Fixes: c68314dd4263 ("src: infer NAT mapping with concatenation from set")
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>