evaluate: check if set exists before listing it

evaluate: check if set exists before listing it

After this patch, we obtain:

 # nft list set ip6 test pepe
 <cmdline>:1:1-22: Error: Could not process rule: Set 'foo' does not exist
 list set ip6 test foo
 ^^^^^^^^^^^^^^^^^^^^^

So we get things aligned with table and chain listing commands.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>