bridge: guard local VLAN-0 FDB helpers against NULL vlan group

In the Linux kernel, the following vulnerability has been resolved:

bridge: guard local VLAN-0 FDB helpers against NULL vlan group

When CONFIG_BRIDGE_VLAN_FILTERING is not set, br_vlan_group() and
nbp_vlan_group() return NULL (br_private.h stub definitions). The
BR_BOOLOPT_FDB_LOCAL_VLAN_0 toggle code is compiled unconditionally and
reaches br_fdb_delete_locals_per_vlan_port() and
br_fdb_insert_locals_per_vlan_port(), where the NULL vlan group pointer
is dereferenced via list_for_each_entry(v, &vg->vlan_list, vlist).

The observed crash is in the delete path, triggered when creating a
bridge with IFLA_BR_MULTI_BOOLOPT containing BR_BOOLOPT_FDB_LOCAL_VLAN_0
via RTM_NEWLINK. The insert helper has the same bug pattern.

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000056: 0000 [#1] KASAN NOPTI
KASAN: null-ptr-deref in range [0x00000000000002b0-0x00000000000002b7]
RIP: 0010:br_fdb_delete_locals_per_vlan+0x2b9/0x310
Call Trace:
br_fdb_toggle_local_vlan_0+0x452/0x4c0
br_toggle_fdb_local_vlan_0+0x31/0x80 net/bridge/br.c:276
br_boolopt_toggle net/bridge/br.c:313
br_boolopt_multi_toggle net/bridge/br.c:364
br_changelink net/bridge/br_netlink.c:1542
br_dev_newlink net/bridge/br_netlink.c:1575

Add NULL checks for the vlan group pointer in both helpers, returning
early when there are no VLANs to iterate. This matches the existing
pattern used by other bridge FDB functions such as br_fdb_add() and
br_fdb_delete().