NCBC-330 - During finalization a NRE is thrown on SocketPool 

Fixed minor bug where _pool is null during finalization

Change-Id: I6858811535cbdf1f510ebaf963a0668ec79167d7
Reviewed-on: http://review.couchbase.org/30236
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>