The ornamental design for a lamp, as shown and described.
FIG. 1 is a front perspective view of a lamp showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof; and,
In FIG. 8, the broken line circles and the portions within the broken line circles illustrate portions of the lamp which form no part of the claimed design.
Grayscale shading illustrates contour, not any particular color.