How to free this sewing pin from the finished shirt
Try crushing the glass head with a pair of pliers. If you're lucky, the metal head inside the glass is small enough to be removed without opening the seam.

If it's still stuck, cut the head of with wirecutters like Henry Taylor described. Keep in mind that the needle head may rust after being washed several times. You should tuck the left-over head into a corner that's as invisible as possible.