DETROIT — It’s a case of buyer beware, with potentially dangerous consequences. More than 46 million cars and trucks on the road in the U.S. — about one-fifth the total — were recalled because of safety defects but never repaired, according to a study by Carfax, a company that sells vehicle history reports. Some of those defects have the potential to cause a crash, injury, even death. Last year, around 5 million of those cars were sold to new owners. That’s because there is no legal …