tipc: add a bounds check in link_recv_changeover_msg()

The bearer_id here comes from skb->data and it can be a number from 0 to
7.  The problem is that the ->links[] array has only 2 elements so I
have added a range check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>