netfilter: nft_osf: check if attribute is present

If the attribute is not sent, eg. old libnftnl binary, then
tb[NFTA_OSF_TTL] is NULL and kernel crashes from the _init path.

Fixes: a218dc82 ("netfilter: nft_osf: Add ttl option support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_osf.c

@@ -50,7 +50,7 @@ static int nft_osf_init(const struct nft_ctx *ctx,
 	int err;
 	u8 ttl;
 
-	if (nla_get_u8(tb[NFTA_OSF_TTL])) {
+	if (tb[NFTA_OSF_TTL]) {
 		ttl = nla_get_u8(tb[NFTA_OSF_TTL]);
 		if (ttl > 2)
 			return -EINVAL;