Fixed #31683 : In debug mode with mysqli some extensions have warnings when freeing mysqli cursor themselves

Really only a warning suppression in debug mode only.

Some extension do their own mysqli handling and free the result without using the Joomla function (as it is protected it can't be used).

Thus this fix is really needed.

Related tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31683