A former West Mifflin policeman was sentenced to six to 12 months in jail today, and a judge said he made the sentence longer because of allegations the defendant's representatives harassed a juror. Allegheny County Judge David Cashman issued the sentence for Noel Missig, a 14-year police veteran who was convicted of simple assault on a juvenile and perjury when he later said the assault did not happen. Mr. Missig also was convicted of lying to obtain a search warrant of a borough business.