The ornamental design for a lamp, as shown and described.
FIG. 1 is an isometric view of an embodiment of a lamp according to my design;
FIG. 2 is front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right elevational side view thereof;
FIG. 5 is a left elevational side view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
Broken lines having repeated short dashes in the figures are for illustrative purposes only and form no part of the claimed design. Shade lines having a combination of long and short lines in the figures show contour and not surface ornamentation.