evaluate: bogus error when adding devices to flowtable

evaluate: bogus error when adding devices to flowtable

Bail out if flowtable declaration is missing and no devices are
specified.

Otherwise, this reports a bogus error when adding new devices to an
existing flowtable.

 # nft -v
 nftables v1.0.9 (Old Doc Yak #3)
 # ip link add dummy1 type dummy
 # ip link set dummy1 up
 # nft 'create flowtable inet filter f1 { hook ingress priority 0; counter }'
 # nft 'add flowtable inet filter f1 { devices = { dummy1 } ; }'
 Error: missing hook and priority in flowtable declaration
 add flowtable inet filter f1 { devices = { dummy1 } ; }
                           ^^^^^^^^^^^^^^^^^^^^^^^^

Fixes: 5ad475fce5a1 ("evaluate: bail out if new flowtable does not specify hook and priority")
Reported-by: Martin Gignac <martin.gignac@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>