The ornamental design for a winch drum, as shown and described.
FIG. 1 is a perspective view of one embodiment of the invention;
FIG. 2 is an end view with the opposite end being a mirror image thereof;
FIG. 3 is a side view thereof with all sides being the same;
FIG. 4 is a perspective view of another embodiment of the present invention;
FIG. 5 is an end view thereof;
FIG. 6 is an end view of the opposite end of FIG. 5; and,
FIG. 7 is a side view thereof with all sides being the same.
The broken line showing a handle at some elements in some of the figures is included for the purpose of illustrating environment and forms no part of the claimed design.