The ornamental design for a lamp, as shown and described.
FIG. 1 is a front, top, right-side perspective view of a lamp showing my new design, the bottom being plain, unornamented and not visible in use;
FIG. 2 is a rear, top, left-side perspective view thereof;
FIG. 3 is a front, top, right-side perspective view thereof with circular rib surrounding the central LED orifice and serving as a base for an illuminated object; and,
FIG. 4 is a front, top, right-side perspective view thereof with circular rib with the object is shown in a dot-dash line.