tipc: add NULL pointer check to prevent kernel oops

Calling:
tipc_node_link_down()->
   - tipc_node_write_unlock()->tipc_mon_peer_down()
   - tipc_mon_peer_down()
  just after disabling bearer could be caused kernel oops.

Fix this by adding a sanity check to make sure valid memory
access.

Acked-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: Hoang Le <hoang.h.le@dektech.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>