The ornamental design for a potholder, as shown and described.
FIG. 1 is a top/front perspective view of the pot holder;
FIG. 2 is a top plan view of the pot holder;
FIG. 3 is a bottom plan view of the pot holder, the elliptical shapes on the bottom of the pot holder shown in broken line forming no part of the claimed design;
FIG. 4 is a front side elevation view of the pot holder;
FIG. 5 is a rear side elevation view of the pot holder;
FIG. 6 is a right side elevation view of the pot holder; and,