net: bridge: switchdev: check br_vlan_group() return value

br_vlan_group() can return NULL and thus return value must be checked
to avoid dereferencing a NULL pointer.

Fixes: 6284c723d9 ("net: bridge: mst: Notify switchdev drivers of VLAN MSTI migrations")
Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20220421101247.121896-1-clement.leger@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/bridge/br_switchdev.c

@@ -353,6 +353,8 @@ static int br_switchdev_vlan_attr_replay(struct net_device *br_dev,
 	attr.orig_dev = br_dev;
 
 	vg = br_vlan_group(br);
+	if (!vg)
+		return 0;
 
 	list_for_each_entry(v, &vg->vlan_list, vlist) {
 		if (v->msti) {