Occasional error in MongoReplicaSetClient.alive() exposed by test_alive in test_ha.py:

  File "/Users/emptysquare/.virtualenvs/official/mongo-python-driver/pymongo/mongo_replica_set_client.py", line 1265, in alive
    member.pool.maybe_return_socket(sock_info)
AttributeError: 'NoneType' object has no attribute 'pool'

If the replica set has no primary, alive() still tries to return its sock_info to the primary member's pool, even though both sock_info and member are None.