The ornamental design for a casket, as shown and described.
FIG. 1 is a perspective view of a casket showing my new design;
FIG. 2 is a left side view of thereof, the right side view being identical to the left side view;
FIG. 3 is a front view thereof, the rear view being identical to the front view;
FIG. 4 is a top view thereof; and,
FIG. 5 is a bottom view thereof.
The dashed line showing of structural lines is included for the purpose of illustrating non-claimed subject matter and forms no part of the claimed design.