netfilter: nft_osf: add missing check for DREG attribute

commit 7eaecf79 upstream.

syzbot reports just another NULL deref crash because of missing test
for presence of the attribute.

Reported-by: syzbot+cf23983d697c26c34f60@syzkaller.appspotmail.com
Fixes:  b96af92d ("netfilter: nf_tables: implement Passive OS fingerprint module in nft_osf")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/netfilter/nft_osf.c

@@ -47,6 +47,9 @@ static int nft_osf_init(const struct nft_ctx *ctx,
 	struct nft_osf *priv = nft_expr_priv(expr);
 	int err;
 
+	if (!tb[NFTA_OSF_DREG])
+		return -EINVAL;
+
 	priv->dreg = nft_parse_register(tb[NFTA_OSF_DREG]);
 	err = nft_validate_register_store(ctx, priv->dreg, NULL,
 					  NFT_DATA_VALUE, NFT_OSF_MAXGENRELEN);