The ornamental design for a lamp, as shown and described.
FIG. 1 is a perspective view from the left side of the front of a lamp showing my design, the portions of the lamp shown in broken lines are for the purpose of illustration and form no part of the claimed design;
FIG. 2 is a perspective view from the left side of the rear thereof;
FIG. 3 is a perspective view from the right side of the rear thereof; and,
FIG. 4 is a perspective view from the top of the rear of the lamp, in an alternate position.