ebtables: Exit gracefully on invalid table names

ebtables: Exit gracefully on invalid table names

Users are able to cause program abort by passing a table name that
doesn't exist:

| # ebtables-nft -t dummy -P INPUT ACCEPT
| ebtables: nft-cache.c:455: fetch_chain_cache: Assertion `t' failed.
| Aborted

Avoid this by checking table existence just like iptables-nft does upon
parsing '-t' optarg. Since the list of tables is known and fixed,
checking the given name's length is pointless. So just drop that check
in return.

With this patch in place, output looks much better:

| # ebtables-nft -t dummy -P INPUT ACCEPT
| ebtables v1.8.7 (nf_tables): table 'dummy' does not exist
| Perhaps iptables or your kernel needs to be upgraded.

Signed-off-by: Phil Sutter <phil@nwl.cc>