fix the logic of api.lbaas._pool_list

When the subnet is deleted first, _pool_list will access cidr from
a None object. As a result, a unexpected exception is thrown.

Change-Id: Iceb9dea1b0e8bab9ca54dfcdd2bf664c8677e0f2
Related-Bug: #1367615
Merge "fix the logic of api.lbaas._pool_list"