Red-hot "Turner and Hooch," starring Tom Hanks and that slobbering pooch, may outfight some strong competition and be the next No. 1 rental on the Billboard chart. It has already proved it has the muscle--bolting to No. 2 in just two weeks. But strong Valentine's Day business may keep "When Harry Met Sally . . . "--the most popular romantic comedy since "Moonstruck"--on top for another week or two. So far, "Uncle Buck," the comedy starring John Candy, isn't performing like a movie that raked in $63 million. In its second week, "Buck" waddled up only two rungs--to No. 11. "The Karate Kid Part III" (No. 10) is doing good rental business. Jean-Claude Van Damme--the John Wayne of martial arts flicks--has another smash with "Kickboxer," which grossed an impressive $14 million at the box office. On the rental chart, "Kickboxer" bounded up to No. 12 in just two weeks.