We claim the ornamental design for a mattress, as shown and described.
FIG. 1 is a top perspective view of a mattress of the present invention.
FIG. 2 is a top view of the mattress of FIG. 1.
FIG. 3 is a right side view of the mattress of FIG. 1.
FIG. 4 is a left side view of the mattress of FIG. 1
FIG. 5 is a right end view of the mattress of FIG. 1 with the left end being a mirror image thereof; and,
FIG. 6 is a bottom view of the mattress of FIG. 1.
The portions of the mattress shown in broken lines are included for the purpose of illustrating environment and form no part of the claimed design. The portions of the body support shown in broken lines illustrate the boundary of the claimed design and form no pan of the claimed design.