I'm sure a modest lease extension could be agreed if need be. It's in nobody's interests to kick the football club out if there's an overrun of a few months or so. The Malkinsons will still get their cash for the land in the long run, it would be pretty callous of them to force the club into utter turmoil for the sake of 3-6 months.
That said, if anything happens to derail the stadium project for longer than that then it would admittedly be a little more problematic.
Not that I know the Malkies, of course, but I'd hope common sense would prevail...