drivers/net/bonding/bond_3ad: return when there's no aggregator

Otherwise we would dereference a NULL aggregator pointer when calling
__set_agg_ports_ready on the line below.

Found by Linux Verification Center (linuxtesting.org) with the SVACE
static analysis tool.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>