xfrm: fix possible derferencing in error path

Fix derferencing pointer when xfrm_policy_lookup_bytype returns an
 error.

Fixes: 63b21cab ("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>