The Catholic Church does not attribute infallibility to saints. Only Popes and the bishops in union with him are infallible, and only when teaching the content of the Faith which must be believed by everyone, always, and everywhere.
Most saints have not been theologians and were not involved in a teaching office—i.e., they have not been a pope or a bishop. St. Thomas Aquinas, generally considered the greatest theologian of all time, held a number of views that the Church later rejected. Most famously, he did not affirm the Immaculate Conception, which was widely held but not solemnly defined at the time.
Moreover, the prophecies of St. Malachy cannot be infallible because they have nothing to do with the content of the Faith.
They don't have to be infallible...The question is: are they accurate???