The ornamental design for a lamp, as shown and described.
FIG. 1 is a front, left perspective view of a first embodiment of a lamp;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The dashed lines shown illustrate portions of the lamp that form no part of the claimed design. The dot-dash-dot lines immediately adjacent the shaded area represent boundary lines.