evaluate: check for concatenation in set data datatype

evaluate: check for concatenation in set data datatype

When adding this rule with an existing map:

  add rule nat x y meta l4proto { tcp, udp } dnat ip to ip daddr . th dport map @fwdtoip_th

reports a bogus:

Error: datatype mismatch: expected IPv4 address, expression has type
concatenation of (IPv4 address, internet network service)

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>