We claim the ornamental design for a ball rack, as shown and described.
FIG. 1 is front view of a ball rack;
FIG. 2 is rear view of the ball rack;
FIG. 3 is a side view of the ball rack;
FIG. 4 is a top view of the ball rack;
FIG. 5 is a bottom view of the ball rack; and,
FIG. 6 is a perspective view of the ball rack.
Applicant notes that the side view of the ball rack as shown in FIG. 3 is representative of both a right side view and left side view of the ball rack. Any subject matter shown in dashed lines, and any surfaces of the design not shown, including interior surfaces, form no part of the claimed invention.