netfilter: nft_tunnel: fix null-attribute check

else we get null deref when one of the attributes is missing, both
must be non-null.

Reported-by: syzbot+76d0b80493ac881ff77b@syzkaller.appspotmail.com
Fixes: aaecfdb5 ("netfilter: nf_tables: match on tunnel metadata")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_tunnel.c

@@ -76,7 +76,7 @@ static int nft_tunnel_get_init(const struct nft_ctx *ctx,
 	struct nft_tunnel *priv = nft_expr_priv(expr);
 	u32 len;
 
-	if (!tb[NFTA_TUNNEL_KEY] &&
+	if (!tb[NFTA_TUNNEL_KEY] ||
 	    !tb[NFTA_TUNNEL_DREG])
 		return -EINVAL;