General Motors' (GM) common stock "is probably not worth anything," said Jim Cramer on Monday's "Stop Trading!" segment on CNBC.Former CEO Rick Wagoner's "whole goal was to preserve the common stock, and that's just not the ballgame," said Cramer. Cramer said he struggles with whether he should say on air that the common stock is worthless because "for those who are speculating on it, if it goes to $3, they'll say, 'Well, why did you keep me out of it?'" "It's a nice lottery ticket," he said. "Maybe somebody feels that the common stock prevails and it goes to the unions not the banks." He said he doesn't think that will happen, but he's "wary" to say people shouldn't speculate on it. "That's part of what they do," he said.