North Korea wouldn't be able to move its forces into Russia. They do technically share a short land border that has a rail connection that would be pretty trivial for Russia to defend. It would be like trying to invade the US by landing in Alaska. And Russia could almost certinly eliminte DPRK's forces quickly. And even if they made it to Moscow, Russia is likely to use nukes if they feel they're under true threat of being overthrown. While many nukes would fail, many probably would not fail.

Also, China would come down hard on North Korea's stupidity.