The ornamental design for a set-top box, as shown and described.
FIG. 1 is a perspective view of our new design for a set-top box;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is an alternate perspective view thereof showing the components of the set-top box separately.
The broken phantom lines depict unclaimed subject matter only and form no part of the claimed design. The bottom view forms no part of the claimed design.