Whenever DeShaun Foster takes a handoff, 49ers running-backs coach Tony Nathan starts sounding like a pitching coach. “High and tight. High and tight,” Nathan tells Foster. He is reminding Foster how to hold the football. The recently acquired backup running back has a history of fumbles – he lost five of seven last season – but the 49ers think they can help him get a grip. Foster, a five-year veteran, understands the concern. “It’s part of the game,” he said Tuesday after the 49ers’ sixth of 12 organized team activities. “I tend to swing the ball a little bit low when I’m trying to make