"Hey, Osama! Long time no see! I have a few nukes lying around, you want one?"
A common scare scenario put forth by warmongers, but highly unlikely. States with fixed, easily targeted assets that build nuclear weapons do so at considerable expense, and do not just hand them off to stateless terrorists, many of whom are as much a threat to that state as they are to that state's enemies.
Saddam Hussein, for example, would've never in a million years given or sold any nuclear weapons (which he was
not building) to such a F-wit as bin Laden, since bin Laden was a huge threat to the Iraqi state and to Saddam's regime.
In addition, giving such a weapon away to such an irresponsible terrorist would almost certainly result in its being used on someone like, say Isreal or the US, and if it was traced back to him he knew damn well there'd be hell to pay.
The North Koreans are no different; they won't do anything that threatens their regime.