The ornamental design for a lamp, as shown and described.
FIG. 1 is a perspective view of the lamp of my design. The broken lines representing stitching are a part of the claimed design;
FIG. 2 is a top view of the lamp. The broken lines representing stitching are a part of the claimed design;
FIG. 3 is a bottom view of the lamp. The broken lines representing a power cord form no part of the claimed design;
FIG. 4 is a front view of the lamp (the back side view, being a mirror image of the front view, is not shown); and,
FIG. 5 is a right side view of the lamp (the left side view, being a mirror image of the right side view, is not shown). The broken lines representing stitching are a part of the claimed design.