[IPSEC]: Check x->encap before dereferencing it

We need to dereference x->encap before dereferencing it for encap_type.
If it's absent then the encap_type is zero.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/xfrm4_input.c

@@ -90,7 +90,7 @@ int xfrm4_rcv_encap(struct sk_buff *skb, __u16 encap_type)
 		if (unlikely(x->km.state != XFRM_STATE_VALID))
 			goto drop_unlock;
 
-		if (x->encap->encap_type != encap_type)
+		if ((x->encap ? x->encap->encap_type : 0) != encap_type)
 			goto drop_unlock;
 
 		if (x->props.replay_window && xfrm_replay_check(x, seq))