xfrm: NULL dereference on allocation failure

The default error code in pfkey_msg2xfrm_state() is -ENOBUFS.  We
added a new call to security_xfrm_state_alloc() which sets "err" to zero
so there several places where we can return ERR_PTR(0) if kmalloc()
fails.  The caller is expecting error pointers so it leads to a NULL
dereference.

Fixes: df71837d5024 ("[LSM-IPSec]: Security association restriction.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/key/af_key.c

@@ -1157,6 +1157,7 @@ static struct xfrm_state * pfkey_msg2xfrm_state(struct net *net,
 			goto out;
 	}
 
+	err = -ENOBUFS;
 	key = ext_hdrs[SADB_EXT_KEY_AUTH - 1];
 	if (sa->sadb_sa_auth) {
 		int keysize = 0;