WASHINGTON — When news broke about the fall of President Bashar al-Assad’s government in Syria this week, photojournalist Paul Conroy found himself thinking about a former colleague, Marie Colvin. An American journalist, Colvin was killed in February 2012 in the city of Homs when the Syrian military shelled the media center she was in. French photographer Remi Ochlik was also killed, and Conroy and two other …