The ornamental design for a lamp, as shown and described.
FIG. 1 is a top, front, and left side perspective view of a lamp showing my new design;
FIG. 2 is a top, rear, and right side perspective view thereof;
FIG. 3 is a top, front, and left side perspective view thereof, with the shade portion omitted, and the broken line disclosure being understood to show portions of the article in which the claimed design is embodied, but which forms no part of the claimed design; and,
FIG. 4 is an additional top, front, and left side perspective view thereof.