It will be at least eight-and-a-half years behind bars for the Wal-Mart Stalker.   34-year-old Seth Griffith of Norristown was sentenced Friday for following a woman home from the West Norriton store and attempting to rape her.   During his trial, he claimed the 19-year-old victim smiled at him and tugged at her shorts, an action that he took as an invitation for a sexual tryst.   The judge that sentenced Griffith called him a predator and a danger to society.   The judge also said danger lurked deep inside of him.   Griffith remained stone faced and said little during his sentencing.