The ornamental design for a lamp, as shown and described.
FIG. 1 is a top, front, and right side perspective view of a lamp showing our new design;
FIG. 2 is a right side elevational view thereof;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom view thereof.
The area between the ends of the elements shown broken away forms no part of the claimed design, and the portions of the lamp shown in broken lines form no part of the claimed design.