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 bottom perspective view thereof;
FIG. 3 is a first side view thereof;
FIG. 4 is a second side view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a rear view thereof;
FIG. 7 is a top view thereof;
FIG. 8 is a bottom view thereof;
FIG. 9 is an enlarged view of the portion delineated by box ‘9’ of FIG. 3;
FIG. 10 is an enlarged view of the portion delineated by box ‘10’ of FIG. 8; and,
FIG. 11 is an enlarged view of the portion delineated by box ‘11’ of FIG. 2.
The evenly spaced broken lines represent unclaimed subject matter and form no part of the claimed design. Additionally, the dot-dash broken lines in each of FIGS. 2, 3 and 8-ll define the section in which the enlarged view is taken and form no part of the claimed design.