We claim the ornamental design for a mattress tub, as shown and described.
FIG. 1 is a front, top perspective view of a mattress tub showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The mattress tub is shown with symbolic breaks using dot-dashed, broken lines. The appearance of any portion of the mattress tub between the dot-dashed, broken lines forms no part of the claimed design. The evenly spaced, broken lines represent the portions of the mattress tub that form no part of the claimed design.