INSTEAD OF A bank vault or a Brink's truck, the betting slips from what could be the largest payout in sports gambling history are being guarded by nothing more than an old, tattered Houston Astros backpack. On a recent Tuesday afternoon in Houston, with the MLB playoffs about to begin, the faded blue nylon bag -- its contents worth potentially millions -- sits on the floor of the massive Gallery Furniture showroom, just within arm's reach of its owner: the Houston furniture magnate and Astros superfan Jim McIngvale, better known around these parts and in the world of high-stakes sports gambling as Mattress Mack.