The ornamental design for a diaper, as shown and described.
FIG. 1 is a top plan view of a diaper showing our new design;
FIG. 2 is a top plan view of another embodiment thereof; and,
FIG. 3 is a top plan view of another embodiment thereof.
The broken line showing of a diaper is included for the purpose of illustrating environmental structure and forms no part of the claimed design.