Six investors in Lancelot Investors Fund — a Cayman Islands hedge fund that got suckered by the operator of a billion-dollar Ponzi scheme — sued Lancelot’s auditors in Cook County, where the offshore company is headquartered, alleging that fraudulent and negligent audit reports duped them into pumping $79 million into Lancelot from 2004 to 2008, when the pyramid scheme imploded, wiping out their investment.But the trial judge — concluding that the internal-affairs doctrine required application of …