The ornamental design for a stemware accessory, as shown and described.
FIG. 1 is a perspective view of a stemware accessory showing my new design, the glass is shown in broken lines for illustrative purposes only and forms no part of the claimed design;
FIG. 2 is a side view of the stemware accessory of the present invention;
FIG. 3 is top view of the stemware accessory of the present invention; and,
FIG. 4 is a bottom view of the stemware accessory of the present invention.