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

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

After commit a07e4130d858d67977e67d18cca0fd6269b3198e ("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>