We claim the ornamental design for a ball bat, as shown and described.
FIG. 1 is a side perspective view of a ball bat of the present design;
FIG. 2 is a side view of the ball bat of FIG. 1; and,
FIG. 3 is an end view of the ball bat of FIG. 1.
The ornamental design that is claimed is shown in solid lines in the drawings. The broken (dashed) lines shown in FIGS. 1 through 3 are included only for the purpose of illustrating environmental elements and break lines, and form no part of the claimed design.