The ornamental design for a dumbbell, as shown and described.
FIG. 1 is a perspective view of a dumbbell according to the design of the invention.
FIG. 2 is a front view of one of the two weights from the dumbbell, the rear, top, and bottom views of the weight being identical. The appearance of the second weight is identical thereto.
FIG. 3 is a view of the outer end of the dumbbell, both outer ends being identical; and,
FIG. 4 is a view of the inner end of one of the weights, the appearance being identical for both weights.
The handle of the dumbbell is shown in broken lines because it forms no part of the claimed design.