FIG. 1 is an isometric view of a front, top, and right side of a first embodiment of an ornamental design for a mattress;
FIG. 2 is an elevational view of the front side of the mattress shown in FIG. 1, a rear being identical to the front;
FIG. 3 is an elevational view of the left side of the mattress shown in FIG. 1, a right side being identical to the left side;
FIG. 4 is a top view of the mattress shown in FIG. 1;
FIG. 5 is a bottom view of the mattress shown in FIG. 1;
FIG. 6 is an isometric view of a front, top, and right side of a second embodiment of an ornamental design for a mattress;
FIG. 7 is an elevational view of the front side of the mattress shown in FIG. 6, a rear being identical to the front;
FIG. 8 is an elevational view of the left side of the mattress shown in FIG. 6, a right side being identical to the left side;
FIG. 9 is a top view of the mattress shown in FIG. 6; and,
FIG. 10 is a bottom view of the mattress shown in FIG. 6.
Any broken lines in the drawings depict environmental subject matter only and form no part of the claimed design.