The ornamental design for a mattress, as shown and described.
FIG. 1 is a perspective view of a mattress, showing my design.
FIG. 2 is a front elevation view of the mattress of FIG. 1, a rear elevation view of the mattress of FIG. 1 being symmetrical to the front side elevation view of the mattress of FIG. 1.
FIG. 3 is a left side elevation view of the mattress of FIG. 1, a right side elevation of the mattress of FIG. 1 being symmetrical to the left side elevation view of the mattress of FIG. 1.
FIG. 4 is a top plan view of the mattress of FIG. 1, a bottom plan view of the mattress of FIG. 1 being symmetrical to the top plan view of the mattress of FIG. 1; and,
FIG. 5 is an enlarged view of an area labeled 5 in FIG. 1.
The broken lines in the drawings depict environmental structure and boundaries and form no part of the claimed design.