bonding: fix bond_inet6addr_event()

After commit ad1afb00 (vlan_dev: VLAN 0 should be treated
as "no vlan tag" (802.1p packet)),
bond_inet6addr_event() might be called with a NULL bond->vlgrp pointer, and
a non empty bond->vlan_list. vlan_group_get_device() is dereferencing a NULL pointer.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>