The ornamental design for a set-top box, as shown and described.
FIG. 1 is a perspective view of a set-top box according to an aspect of the disclosure;
FIG. 2 is a front view of a set-top box according to an aspect of the disclosure;
FIG. 3 is a top view of a set-top box according to an aspect of the disclosure;
FIG. 4 is a rear view of a set-top box according to an aspect of the disclosure;
FIG. 5 is a side view of a set-top box according to an aspect of the disclosure; and,
FIG. 6 is another side view of a set-top box according to an aspect of the disclosure.
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.