I claim the ornamental design for a cup holder adaptor, as shown and described.
FIG. 1 is a perspective top, front view of a cup holder adaptor including a cup holder supported container in phantom, wherein the phantom lines illustrating the cup holder supported container do not form any part of the design;
FIG. 2 is a perspective top, front view of the cup holder adaptor;
FIG. 3 is a front view thereof;
FIG. 4 is a rotated side view thereof, being 45 degrees from the front view;