bridge: vlan: fix possible null vlgrp deref while registering new port

While a new port is being initialized the rx_handler gets set, but the
vlans get initialized later in br_add_if() and in that window if we
receive a frame with a link-local address we can try to dereference
p->vlgrp in:
br_handle_frame() -> br_handle_local_finish() -> br_should_learn()

Fix this by checking vlgrp before using it.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>