The ornamental design for a socket, as shown and described.
FIG. 1 is a front-right-top perspective view of a socket showing my new design;
FIG. 2 is a front elevational view thereof (the rear elevational view is a mirror image of the front elevational view, the right side elevational view is a mirror image of the front elevational view, and the left side elevational view is a mirror image of the front elevational view);
FIG. 3 is a top plan view thereof; and,
FIG. 4 is a bottom plan view thereof.
The broken lines in the drawings depict environmental subject matter only and form no part of the claimed design.