I claim the ornamental design for a bed, as shown and described.
FIG. 1 is a front perspective view of a bed according to the invention. The view from the adjacent front corner and the adjacent rear corner being a mirror image thereto, and the view from the opposite rear corner being identical thereto; and,
FIG. 2 is a front end view of the bed depicted in FIG. 1. The view from the rear end being identical thereto.