Backported fix from 3.x to fix JHttpTransportStream on 64bit systems

See: 
3.x tracker item: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=29565&start=0
2.5.x tracker item: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32683&start=0