netfilter: xt_nfacct: don't assume acct name is null-terminated

[ Upstream commit bf58e667af7d96c8eb9411f926a0a0955f41ce21 ]

BUG: KASAN: slab-out-of-bounds in .. lib/vsprintf.c:721
Read of size 1 at addr ffff88801eac95c8 by task syz-executor183/5851
[..]
 string+0x231/0x2b0 lib/vsprintf.c:721
 vsnprintf+0x739/0xf00 lib/vsprintf.c:2874
 [..]
 nfacct_mt_checkentry+0xd2/0xe0 net/netfilter/xt_nfacct.c:41
 xt_check_match+0x3d1/0xab0 net/netfilter/x_tables.c:523

nfnl_acct_find_get() handles non-null input, but the error
printk relied on its presence.

Reported-by:  <syzbot+4ff165b9251e4d295690@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=4ff165b9251e4d295690


Tested-by:  <syzbot+4ff165b9251e4d295690@syzkaller.appspotmail.com>
Fixes: ceb98d03 ("netfilter: xtables: add nfacct match to support extended accounting")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>