Just 20 years earlier, the 'corpse of Poland' was supposed to be the road to global revolution. The defeat of the Soviet army at Warsaw in 1920 was a major setback to international socialism. Also the territories Poland took in that war, while historically a part of the Polish-Lithuanian Commonwealth, were populated largely by what we would call today Belarusians and Ukrainians. These groups were seen as part of a greater russian ethnicity, who were vulnerable to German atrocities due to the failure of the Polish government to protect them. This supposed intervention was the official reason cited by the soviets, though I would add that the sentiment of Poland as a historical enemy of international socialism helped as well.

Edit: after rereading your question, I would like to note that the molotov-ribbentrop pact was secret, and no 'justification of it was necessary. Apologies, I initially read your question as asking about the justification for invasion, not the pact itself.