The ornamental design for a cable drum, as shown and described.
FIG. 1 is a front, right hand side perspective view of a cable drum embodying the principles of my new design;
FIG. 2 is a rear, left hand side perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a side view thereof as seen from the right hand side of FIG. 3, the opposite side view being a mirror image thereof; and,
FIG. 6 is a front, right hand side perspective view of the cable drum shown in a frame.
The broken lines immediately adjacent the solid line portions of the drawing figures represent the bounds of the claimed design, while all other broken lines depict portions of the cable drum that form no part of the claimed design.