The ornamental design for a tub, as shown and described.
FIG. 1 is a perspective view of a tub showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a first side view thereof, the opposite side being a mirror image of the side shown;
FIG. 4 is a bottom view thereof; and,
FIG. 5 is a first end view thereof, the opposite end being a mirror image of the end shown.
The broken lines showing a plumbing drain hole are included for the purpose of illustrating a portion of the tub that forms no part of the claimed design.