evaluate: bail out with concatenations and singleton values

evaluate: bail out with concatenations and singleton values

The rule:

 # nft add rule x y iifname . oifname p . q

is equivalent to:

 # nft add rule x y iifname p oifname q

Bail out with:

 Error: Use concatenations with sets and maps, not singleton values
 add rule x y iifname . oifname p . q
              ^^^^^^^^^^^^^^^^^ ~~~~~

instead of:

 BUG: invalid expression type concat
 nft: evaluate.c:1916: expr_evaluate_relational: Assertion `0' failed.
 Aborted

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