xfrm: check for a vaild skb in xfrm_policy_queue_process

We might dreference a NULL pointer if the hold_queue is empty,
so add a check to avoid this.

Bug was introduced with git commit a0073fe1 ("xfrm: Add a state
resolution packet queue")

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>