A District Court jury in Las Vegas has found San Diego data processor Howard Lee Haupt innocent of a murder charge in the slaying of a 7-year-old California boy at a state-line casino nearly 15 months ago. "I'm very pleased that the justice system works," a visibly shaken Haupt said after the verdict was read. Roxanne Harris, the mother of victim Alexander Harris, sobbed loudly and put her head in her hands. The jury rendered the verdict after a five-week trial. The Harris boy disappeared Nov. 27, 1987, from Whiskey Pete's casino on the California-Nevada border southwest of Las Vegas. His body was found a month later, and Haupt was arrested in the case a year ago.