The ornamental design for a mattress, as shown and described.
FIG. 1 is an isometric view of a front, top, and right side of an ornamental design for a mattress;
FIG. 2 is a front elevational view of the mattress of FIG. 1, of which the rear elevational view is a mirror image;
FIG. 3 is a right side elevational view of the mattress of FIG. 1, of which the left side elevational view is a mirror image;
FIG. 4 is a top plan view of the mattress of FIG. 1;
FIG. 5 is a bottom plan view of the mattress of FIG. 1; and,
FIG. 6 is an isometric view of a front, top, and right side of another embodiment of an ornamental design for a mattress.
It is understood that the dash-dash-dash broken lines represent a pattern on the surface of the mattress or environmental structure that form no part of the claimed design. The dash-dot-dash broken lines on the mattress of FIGS. 1-5 depict a mattress with a symbolic break in its length and width. The dash-dot-dash broken lines on the mattress of FIG. 6 depict a mattress with a symbolic break in its length, width, and height. The appearance of any portion of the article between the break lines forms no part of the claimed design.