Since commit 222d7dbd ("net: prevent dst uses after free")
skb_dst_force() might clear the dst_entry attached to the skb.
The xfrm code don't expect this to happen, so we crash with
a NULL pointer dereference in this case. Fix it by checking
skb_dst(skb) for NULL after skb_dst_force() and drop the packet
in cast the dst_entry was cleared.

Fixes: 222d7dbd ("net: prevent dst uses after free")
Reported-by: NTobias Hommel <netdev-list@genoetigt.de>
Reported-by: NKristian Evensen <kristian.evensen@gmail.com>
Reported-by: NWolfgang Walter <linux@stwm.de>
Signed-off-by: NSteffen Klassert <steffen.klassert@secunet.com>