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>

net/tipc/monitor.c

@@ -322,9 +322,13 @@ static void mon_assign_roles(struct tipc_monitor *mon, struct tipc_peer *head)
 void tipc_mon_remove_peer(struct net *net, u32 addr, int bearer_id)
 {
 	struct tipc_monitor *mon = tipc_monitor(net, bearer_id);
-	struct tipc_peer *self = get_self(net, bearer_id);
+	struct tipc_peer *self;
 	struct tipc_peer *peer, *prev, *head;
 
+	if (!mon)
+		return;
+
+	self = get_self(net, bearer_id);
 	write_lock_bh(&mon->lock);
 	peer = get_peer(mon, addr);
 	if (!peer)
@@ -407,11 +411,15 @@ void tipc_mon_peer_up(struct net *net, u32 addr, int bearer_id)
 void tipc_mon_peer_down(struct net *net, u32 addr, int bearer_id)
 {
 	struct tipc_monitor *mon = tipc_monitor(net, bearer_id);
-	struct tipc_peer *self = get_self(net, bearer_id);
+	struct tipc_peer *self;
 	struct tipc_peer *peer, *head;
 	struct tipc_mon_domain *dom;
 	int applied;
 
+	if (!mon)
+		return;
+
+	self = get_self(net, bearer_id);
 	write_lock_bh(&mon->lock);
 	peer = get_peer(mon, addr);
 	if (!peer) {