bonding: verify if we still have slaves in bond_3ad_unbind_slave()

After commit 1f718f0 ("bonding: populate
neighbour's private on enslave"), we've moved the unlinking of the slave
to the earliest position possible - so that nobody will see an
half-uninited slave.

However, bond_3ad_unbind_slave() relied that, even while removing the last
slave, it is still accessible - via __get_first_agg() (and, eventually,
bond_first_slave()).

Fix that by verifying if the aggregator return is an actual aggregator, but
not NULL.

CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_3ad.c

@@ -2056,7 +2056,9 @@ void bond_3ad_unbind_slave(struct slave *slave)
 				pr_info("%s: Removing an active aggregator\n",
 					slave->bond->dev->name);
 				// select new active aggregator
-				ad_agg_selection_logic(__get_first_agg(port));
+				temp_aggregator = __get_first_agg(port);
+				if (temp_aggregator)
+					ad_agg_selection_logic(temp_aggregator);
 			}
 		}
 	}