The ornamental design for a casket, as shown and described.
FIG. 1 is a side perspective view of a casket showing my new design;
FIG. 2 is a left side view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a back view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines shown in the drawings depict portions of the casket that form no part of the claimed design. The broken lines shown in FIG. 7 define a region therein that forms no part of the claimed design.