The Infiniband link status change callback ipoib_link_state_changed()
may be called while the IPoIB device is closed, in which case there
will not be an IPoIB queue pair to be joined to the IPv4 broadcast
group.  This leads to NULL pointer dereferences in ib_mcast_attach()
and ib_mcast_detach().

Fix by not attempting to join (or leave) the broadcast group unless we
actually have an IPoIB queue pair.

Signed-off-by: Wissam Shoukair <wissams@mellanox.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>