One possible piece of evidence that suggests perhaps that something will happen to Piggy comes when Ralph pretends to be a fighter plane and machine-guns Piggy. This is hardly a clear bit of foreshadowing but could be considered a possibility.
The first real suggestion comes when Ralph and Piggy are discussing a possible rescue and Piggy, often the only boy who has a clear grip on reality and the specifics of their situation and the outside world, starts to cry about the fact that they may be there "till they die." The fact that Piggy suggests this after going through all the realities of their situation could certainly be considered foreshadowing.
Perhaps the only other suggestion that Piggy is not going to make it is the fact that Ralph has already dismissed him and told the other boys his nickname even though Piggy expressly asked him not to. The fact that Ralph, who represents law and order and civilization, is so willing to dismiss him suggests that Piggy may not last long on the island. Despite (or perhaps because of) his understanding of their situation and that of the rest of the world, he is not seen as particularly useful or important by anyone in a position of leadership.