What is claimed is the ornamental design for a lamp, as shown.
FIG. 1 is a perspective view of the lamp;
FIG. 2 is a front view of the lamp;
FIG. 3 is a back view of the lamp;
FIG. 4 is a right side view of the lamp;
FIG. 5 is a left side view of the lamp;
FIG. 6 is a top view of the lamp;
FIG. 7 is a bottom view of the lamp; and,
FIG. 8 is a perspective view of the lamp.
Note: The portions of the LAMP shown in broken lines form no part of the claimed design. The lamp is shown with symbolic breaks in its length. The appearance of any portion of the article between the break lines form no part of the claimed design.