ST. LOUIS—Thick smoke billowed from the vacant, crumbling St. Louis house, but Benjamin Polson knew homeless people might be inside taking refuge from the January chill. So the 33-year-old firefighter went in. It cost him his life. Eleven days later, three Baltimore firefighters died when an abandoned row home collapsed. The same vacant house was the site of another fire seven years earlier that injured three firefighters. Vacant homes dot the landscape — urban and rural — across the U.S. They’re far more prone to catch fire, and because the structures are often compromised, they are especially dangerous for firefighters. Officials in St. Louis and Baltimore are looking at ways to reduce those risks. St. Louis fire leaders are doing an inventory of every vacant home — all 10,000 of them — with plans to develop a computerized database so firefighters know what they’re getting into. Baltimore Mayor Brandon Scott …