xfrm: avoid possible oopse in xfrm_alloc_dst

Commit 80c802f3 (xfrm: cache bundles instead of policies for
outgoing flows) introduced possible oopse when dst_alloc returns NULL.
Signed-off-by: NHiroaki SHIMODA <shimoda.hiroaki@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/xfrm/xfrm_policy.c

@@ -1340,10 +1340,13 @@ static inline struct xfrm_dst *xfrm_alloc_dst(struct net *net, int family)
 	default:
 		BUG();
 	}
-	xdst = dst_alloc(dst_ops) ?: ERR_PTR(-ENOBUFS);
+	xdst = dst_alloc(dst_ops);
 	xfrm_policy_put_afinfo(afinfo);
 
-	xdst->flo.ops = &xfrm_bundle_fc_ops;
+	if (likely(xdst))
+		xdst->flo.ops = &xfrm_bundle_fc_ops;
+	else
+		xdst = ERR_PTR(-ENOBUFS);
 
 	return xdst;
 }