Fix list index out of range

This patch fixes the following bug:

2014-07-29 14:37:00.174 ERROR neutron.openstack.common.periodic_task
[req-12654a45-70d7-4d88-bf47-26fb2fc3aa62 None None] Error during
DRAgentWithStateReport.periodic_sync_peers_task: list index out of range
Traceback (most recent call last):

File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/rpc/dispatcher.py", line 134, in _dispatch_and_reply
  incoming.message))

File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/rpc/dispatcher.py", line 177, in _dispatch
  return self._do_dispatch(endpoint, method, ctxt, args)

File "/usr/local/lib/python2.7/dist-packages/oslo/messaging/rpc/dispatcher.py", line 123, in _do_dispatch
  result = getattr(endpoint, method)(ctxt, **new_args)

File "/opt/stack/neutron/neutron/services/dynamic_routing/dr_rpc_base.py", line 66, in sync_advertisenetworks
  context, routinginstances[0]['id'])['networks']

IndexError: list index out of range

Change-Id: I5ea6919adf4473899aff082297f92f58540aa2a7
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>