The ornamental design for a lamp, as shown and described.
FIG. 1 is a front elevational view of a lamp showing my new design;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a first perspective view thereof; and,
FIG. 8 is a second perspective view thereof.
The broken lines in the drawings illustrate portions of the lamp which form no part of the claimed design. The diagonal lines in FIG. 1 and FIG. 6 indicate transparency.