The shade lines in the Figures show contour and not surface ornamentation. Broken lines in the Figures show portions of the varistor that form no part of the claimed design. A drawing of a right side view of the varistor is omitted since the drawing of the right side view is identical to a drawing of a left side view of the varistor. A drawing of a rear view of varistor is omitted since the drawing of the rear view is identical to a drawing of a front view of the varistor.