I claim the ornamental design for a lamp, as shown and described.
FIG. 1 is a perspective view of a lamp showing my new design;
FIG. 2 is a front elevational view of FIG. 1, the rear elevational view, right-side elevational view, and left-side elevational view all being identical thereto.
FIG. 3 is a top plan view of FIG. 1; and,
FIG. 4 is a bottom plan view of FIG. 1.
The broken lines showing a power cord, bulb holder, and lightbulb in FIG. 1 depict environmental matter and form no part of the claimed design.