bonding: fix netdev event NULL pointer dereference

In commit 471cb5a3 ("bonding: remove
usage of dev->master") a bug was introduced which causes a NULL pointer
dereference. If a bond device is in mode 6 (ALB) and a slave is added
it will dereference a NULL pointer in bond_slave_netdev_event().
This is because in bond_enslave we have bond_alb_init_slave() which
changes the MAC address of the slave and causes a NETDEV_CHANGEADDR.
Then we have in bond_slave_netdev_event():
        struct slave *slave = bond_slave_get_rtnl(slave_dev);
        struct bonding *bond = slave->bond;
bond_slave_get_rtnl() dereferences slave_dev->rx_handler_data which at
that time is NULL since netdev_rx_handler_register() is called later.

This is fixed by checking if slave is NULL before dereferencing it.

v2: Comment style changed.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>