The ornamental design for a lamp, as shown and described.
FIG. 1 is a perspective view of the lamp;
FIG. 2 is a left side view of the lamp;
FIG. 3 is a right side view of the lamp;
FIG. 4 is a front view of the lamp;
FIG. 5 is a back view of the lamp;
FIG. 6 is a top view of the lamp; and,
FIG. 7 is a bottom view of the lamp.
Elements of the lamp have been broken away in some of the figures to indicate that no particular length is claimed. The portions of the lamp shown in broken lines form no part of the claimed design.