The gun used by the monster who opened fire on a crowded Brooklyn subway Tuesday jammed in the midst of the bloodshed — preventing him from causing more carnage, law enforcement sources told The Post. Investigators recovered the jammed Glock and two extended magazines at the scene of the shooting, which happened aboard a Manhattan-bound N train just as it was pulling into the 36th Street station in Sunset Park just after 8:20 a.m. “It was lucky that it jammed because we could be talking about a lot more people in hospitals or worse,” a law enforcement source said. “Dozens...