The ornamental design for a lamp, as shown and described.
FIG. 1 is a perspective view of a lamp showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines at the base of the lamp in FIGS. 3 and 7 are for the purpose of illustrating a portion of the lamp that forms no part of the claimed design. The remaining broken lines are for the purpose of illustrating environmental structure for the lamp that forms no part of the claimed design.