race condition in DB.requestEnsureConnection can cause NullPointerException

With a replica set connection, use of DB.requestEnsureConnection has a race condition which can cause a NullPointerException:

    java.lang.NullPointerException

	at com.mongodb.DBTCPConnector$MyPort.requestEnsureConnection(DBTCPConnector.java:483)

	at com.mongodb.DBTCPConnector.requestEnsureConnection(DBTCPConnector.java:129)

	at com.mongodb.DBApiLayer.requestEnsureConnection(DBApiLayer.java:110)MongoDBConnection.java:482

This race condition is not actually new, but the NPE happens more frequently now that the driver is handling a list of servers that can be either mongos server or replica set members.