^This.

I think they could very easily justify Katniss having the pin by shifting the backstory from Madge’s mother onto Katniss’s mother. If Mrs. Everdeen had a sister who was a tribute, she could’ve gotten the pin that way and gifted it to Katniss, allowing Katniss to give it to Prim (as seen in the trailer.) Or, if the idea of giving Katniss a tribute aunt is too much, make Maysilee a friend of Mrs. Everdeen who gave her the pin prior to leaving for the arena. Either way, it’s a plausible way of getting the pin (a relatively expensive thing) into Katniss’s hands.

Since it would have sentimental value (and she was catatonic at the time, not really in the position to be providing information and/or goods to Katniss), it wouldn’t have gotten traded while they were starving. 

Or it could be a memento from Katniss’s father, found in the forest or earned through trade. Or a hundred other plausible ways. The background regarding the pin, with the jabberjay, still holds no matter who gave it to Katniss. 

Seriously, I don’t get why this has become such a huge thing. There are a ton of plausible ways a writer can get the pin to Katniss without an unnecessary character.