xfrm: properly handle invalid states as an error

The error exit path needs err explicitly set. Otherwise it
returns success and the only caller, xfrm_output_resume(),
would oops in skb_dst(skb)->ops derefence as skb_dst(skb) is
NULL.

Bug introduced in commit bb65a9cb (xfrm: removes a superfluous
check and add a statistic).
Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Cc: Li RongQing <roy.qing.li@gmail.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/xfrm/xfrm_output.c

@@ -64,6 +64,7 @@ static int xfrm_output_one(struct sk_buff *skb, int err)
 
 		if (unlikely(x->km.state != XFRM_STATE_VALID)) {
 			XFRM_INC_STATS(net, LINUX_MIB_XFRMOUTSTATEINVALID);
+			err = -EINVAL;
 			goto error;
 		}