A freezer malfunction at Harvard-affiliated McLean Hospital has severely damaged one-third of the world’s largest collection of autism brain samples, potentially setting back research on the disorder by years, scientists say. An official at the renowned brain bank in Belmont discovered that the freezer had shut down in late May, without triggering two alarms. Inside, they found 150 thawed brains that had turned dark from decay; about a third of them were part of a collection of autism brains.