I claim the ornamental design for a mattress, as shown and described.
FIG. 1 is a left side, top perspective view of a mattress embodying my design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a left side view thereof, the right side being a mirror image thereof;
FIG. 4 is a foot end view thereof, the head end being a mirror image thereof;
FIG. 5 is a bottom plan view thereof in its intended environment;
FIG. 6 is an enlarged perspective view taken from FIG. 1 showing a portion of the mattress; and,
FIG. 7 is an enlarged side view taken from FIG. 3 showing a portion of the mattress.
The broken line boundaries are environmental and form no part of the claimed design. Accordingly, the broken lines in FIGS. 1, 3, 4, and 5 showing a mattress foundation illustrates the environment and forms no part of the design. The broken lines in FIGS. 6 and 7 showing the boundaries of an enlarged detailed view form no part of the design.