The ornamental design for a mattress, as shown and described.
FIG. 1 is an isometric view of a top, front, and right side of an ornamental design for a mattress;
FIG. 2 is an enlarged detail view of area 2, circumscribed within FIG. 1;
FIG. 3 is a front elevational view of the mattress of FIG. 1;
FIG. 4 is a rear elevational view of the mattress of FIG. 1;
FIG. 5 is a right side elevational view of the mattress of FIG. 1;
FIG. 6 is a left side elevational view of the mattress of FIG. 1;
FIG. 7 is top plan view of the mattress of FIG. 1; and,
FIG. 8 is a bottom plan view of the mattress of FIG. 1.
It is understood that the dashed clover-like surface pattern and the vertical dash-dash-dash-dash broken lines on the sides of the mattress represent a pattern on the surface of the mattress that forms no part of the claimed design. Further, the remaining dash-dash-dash-dash broken lines represent environmental structure that forms no part of the claimed design. The dash-dot-dash-dot broken lines depict a mattress with a symbolic break in its length and width and forms no part of the claimed design. The appearance of any portion of the article between the break lines forms no part of the claimed design. The dotted pattern bounded by the solid lines on the sides depicts a portion of the mattress and forms no part of the claimed design.