ebtables: Fix error message for invalid parameters

ebtables: Fix error message for invalid parameters

With empty ruleset, ebtables-nft would report the wrong argv:

| % sudo ./install/sbin/ebtables-nft -vnL
| ebtables v1.8.3 (nf_tables): Unknown argument: './install/sbin/ebtables-nft'
| Try `ebtables -h' or 'ebtables --help' for more information.

After a (successful) call to 'ebtables-nft -L', this would even
segfault:

| % sudo ./install/sbin/ebtables-nft -vnL
| zsh: segmentation fault  sudo ./install/sbin/ebtables-nft -vnL

Fixes: acde6be32036f ("ebtables-translate: Fix segfault while parsing extension options")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>