netfilter: fix regression in looped (broad|multi)cast's MAC handling

In commit 5648b5e1 ("netfilter: nfnetlink_queue: fix OOB when mac
header was cleared"), the test for non-empty MAC header introduced in
commit 2c38de4c ("netfilter: fix looped (broad|multi)cast's MAC
handling") has been replaced with a test for a set MAC header.

This breaks the case when the MAC header has been reset (using
skb_reset_mac_header), as is the case with looped-back multicast
packets.  As a result, the packets ending up in NFQUEUE get a bogus
hwaddr interpreted from the first bytes of the IP header.

This patch adds a test for a non-empty MAC header in addition to the
test for a set MAC header.  The same two tests are also implemented in
nfnetlink_log.c, where the initial code of commit 2c38de4c
("netfilter: fix looped (broad|multi)cast's MAC handling") has not been
touched, but where supposedly the same situation may happen.

Fixes: 5648b5e1 ("netfilter: nfnetlink_queue: fix OOB when mac header was cleared")
Signed-off-by: Ignacy Gawędzki <ignacy.gawedzki@green-communications.fr>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>