NEW YORK - It was like a classic episode of "Perry Mason," with an FBI agent charged with murder, a mouthy mob moll and a twist at the end: The FBI guy walked after a reporter with a secret tape exposed the gangster's girlfriend as a liar who couldn't keep her story straight. In a stunning finish to one of the worst law enforcement corruption cases in U.S. history, former FBI agent Lindley DeVecchio was cleared Thursday of giving up confidential information that a Colombo family hit man used to kill four fellow mobsters — either rivals or potential rats....