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>

drivers/net/bonding/bond_ipv6.c

@@ -178,6 +178,8 @@ static int bond_inet6addr_event(struct notifier_block *this,
 		}
 
 		list_for_each_entry(vlan, &bond->vlan_list, vlan_list) {
+			if (!bond->vlgrp)
+				continue;
 			vlan_dev = vlan_group_get_device(bond->vlgrp,
 							 vlan->vlan_id);
 			if (vlan_dev == event_dev) {