I very much doubt that Stalin would have been captured as i can see three outcomes if German forces had Moscow encircled.

1. Stalin escapes (Pretty obvious)
2. Stalin commits suicide (i very much doubt it)
3. Stalin is assasinated by the military and Soviets sue for an armistice.

Oh and to Za old uncle Joe was fiercely hated by many in the Soviet Union and he in turn hated and loathed many ethnic groups with in his own country, he and he alone was responsible for conducting many pograms within his country resulting in some circles the deaths of nearly 25 million and if it was not for the KGB and NKDV being his power base for his reign of terror he would have been known as a failed Georgian farmer.