xfrm: fix possible derferencing in error path

Fix derferencing pointer when xfrm_policy_lookup_bytype returns an
 error.

Fixes: 63b21ca ("xfrm: introduce forwarding of ICMP Error messages")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/kernel-janitors/f6ef0d0d-96de-4e01-9dc3-c1b3a6338653@moroto.mountain/
Signed-off-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

Author: antonyantony

net/xfrm/xfrm_policy.c

@@ -3593,6 +3593,8 @@ xfrm_policy *xfrm_in_fwd_icmp(struct sk_buff *skb,
 			return pol;
 
 		pol = xfrm_policy_lookup(net, &fl1, family, XFRM_POLICY_FWD, if_id);
+		if (IS_ERR(pol))
+			pol = NULL;
 	}
 
 	return pol;