The ornamental design for a lamp, as shown and described.
FIG. 1 is a perspective view of the lamp;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a back view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a right side view thereof; and,
FIG. 8 is a section view thereof taken along section line 8-8 of FIG. 2.
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.