The ornamental design for a bed, as shown and described.
FIG. 1 is a perspective view of a bed showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof; and,
FIG. 4 is a right-side view thereof, the left-side view being a mirror image.
The broken lines in the drawings, showing support slats, support member and holes, represent environmental subject matter and form no part of the claimed design.