The ornamental design for a bed, as shown and described.
FIG. 1 is a perspective view of a bed showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof; and,
FIG. 6 is a right side view thereof.
The broken lines are for ornamental purposes only and form no part of the claimed design. The bottom of the bed, which is not shown, forms no part of the claimed design.