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

Fixes: 6284c723 ("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.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>