netfilter: Fix removal of GRE expectation entries created by PPTP

The uninitialized tuple structure caused incorrect hash calculation
and the lookup failed.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=106441
Signed-off-by: Anthony Lineham <anthony.lineham@alliedtelesis.co.nz>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/nf_nat_pptp.c

@@ -45,7 +45,7 @@ static void pptp_nat_expected(struct nf_conn *ct,
 	struct net *net = nf_ct_net(ct);
 	const struct nf_conn *master = ct->master;
 	struct nf_conntrack_expect *other_exp;
-	struct nf_conntrack_tuple t;
+	struct nf_conntrack_tuple t = {};
 	const struct nf_ct_pptp_master *ct_pptp_info;
 	const struct nf_nat_pptp *nat_pptp_info;
 	struct nf_nat_range range;