The ornamental design for a lamp, as shown and described.
FIG. 1 is a top, front, and right side perspective view of a lamp showing my new design;
FIG. 2 is top, rear, and left side perspective view thereof;
FIG. 3 is bottom, front, and left side perspective view thereof; and,
FIG. 4 is a top plan view thereof.
The broken-line disclosure of the four fragmentarily shown cylindrical elements within the shade are understood to be part of the article in which the claimed design is embodied, but which forms no part of the claimed design.