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: Hiroaki SHIMODA <shimoda.hiroaki@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>