A former Little Rock police officer has been convicted on one of five charges related to his escorting - while on a duty - a van that prosecutors say the officer thought was carrying marijuana.    The Arkansas Democrat-Gazette reports that the jury found Randall Robinson guilty Wednesday of lying to investigators. He was acquitted on other charges that include conspiracy to distribute marijuana, attempting to possess marijuana with the intent to distribute and failing to report a crime.    Prosecutors said Robinson's half-brother, former officer Mark Jones, helped arrange the escort of the van. The van was actually part of an undercover FBI investigation.    Robinson's attorney said Robinson was asked by Jones to follow the van but did not know it was supposedly carrying drugs.