evaluate: bail out if anonymous concat set defines a non concat expression

evaluate: bail out if anonymous concat set defines a non concat expression

Iterate over the element list in the anonymous set to validate that all
expressions are concatenations, otherwise bail out.

  ruleset.nft:3:46-53: Error: expression is not a concatenation
               ip protocol . th dport vmap { tcp / 22 : accept, tcp . 80 : drop}
                                             ^^^^^^^^

This is based on a patch from Florian Westphal.

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