PRETORIA, South Africa — The chief prosecutor laughed scornfully at an answer from Oscar Pistorius during the Olympic athlete's murder trial Thursday, mocking the man who shot his girlfriend. The judge sternly instructed the prosecutor to restrain himself and he apologized — then went right back to trying to pick holes in the testimony of the double-amputee runner. It was a harsh day of cross-examination for Pistorius, challenged relentlessly about his account of the moments just before he killed Reeva Steenkamp, as well as circumstances related to several firearms charges against him, including the firing of a gun in a crowded restaurant.