A fire that destroyed the Dependable Cleaners shop on Quincy Avenue last Saturday was accidentally caused by malfunctioning dry cleaning machinery, Quincy fire officials said Tuesday. Fire Chief Joseph Barron said that the malfunction sparked the fire at around 2:15 p.m., and fire officials said the blaze was accelerated by dry cleaning chemicals and hot weather. Insurance agents will determine if the building is a total loss, but Barron estimated that the damage may total as much as $1 million. Christa Hagearty, chief executive of Dependable Cleaners, could not immediately be reached for comment.