The ornamental design for a lamp, as shown and described.
FIG. 1 is a perspective view of a lamp showing my new design, The broken lines in the figures represent portions of the article which form no part of the claim;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a left side elevational view thereof, The broken lines in the figures represent portions of the article which form no part of the claim;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a right side view thereof, The broken lines in the figures represent portions of the article which form no part of the claim; and,
FIG. 7 is a bottom plan view thereof, The broken lines in the figures represent portions of the article which form no part of the claim.