The ornamental design for a mattress, as shown and described.
FIG. 1 is a front, right and top perspective view of a mattress, showing my design.
FIG. 2 is a rear, left and bottom perspective view thereof.
FIG. 3 is a front elevation view thereof.
FIG. 4 is a rear elevation view thereof.
FIG. 5 is a left side elevation view thereof.
FIG. 6 is a right side elevation view thereof.
FIG. 7 is a top plan view thereof.
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is a partial enlarged view of an area labeled 9 in FIG. 1 comprising a corner.
The dash dot dash broken lines shown in FIG. 1 and FIG. 9 are for the purpose of depicting areas of enlargement and form no part of the claimed design.