tipc: fix variable dereference before NULL check

In commit cf6f7e1d ("tipc: dump monitor attributes"),
I dereferenced a pointer before checking if its valid.
This is reported by static check Smatch as:
net/tipc/monitor.c:733 tipc_nl_add_monitor_peer()
     warn: variable dereferenced before check 'mon' (see line 731)

In this commit, we check for a valid monitor before proceeding
with any other operation.

Fixes: cf6f7e1d ("tipc: dump monitor attributes")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Parthasarathy Bhuvaragan <parthasarathy.bhuvaragan@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/monitor.c

@@ -728,12 +728,13 @@ int tipc_nl_add_monitor_peer(struct net *net, struct tipc_nl_msg *msg,
 			     u32 bearer_id, u32 *prev_node)
 {
 	struct tipc_monitor *mon = tipc_monitor(net, bearer_id);
-	struct tipc_peer *peer = mon->self;
+	struct tipc_peer *peer;
 
 	if (!mon)
 		return -EINVAL;
 
 	read_lock_bh(&mon->lock);
+	peer = mon->self;
 	do {
 		if (*prev_node) {
 			if (peer->addr == *prev_node)