The ornamental design for a lamp, as shown and described.
FIG. 1 is a perspective view of a lamp, illustrating my new design;
FIG. 2 is a top plan view of the lamp of FIG. 1;
FIG. 3 is a left elevational view of the lamp of FIG. 1;
FIG. 4 is a right elevational view of the lamp of FIG. 1;
FIG. 5 is a front elevational view of the lamp of FIG. 1; and,
FIG. 6 is a rear elevational view of the lamp of FIG. 1.
In the figures, the unshaded areas enclosed by broken lines form no part of the claim. All other broken lines represent environmental structure which also forms no part of the claim.