I claim the ornamental design for a lamp, as shown.
FIG. 1 is a front elevation view of my new design;
FIG. 2 is a right side elevation view thereof;
FIG. 3 is a left side elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a perspective view from above and left thereof.
The broken-line disclosure in the views is understood to represent a portion of the article in which the claimed design is embodied, but which forms no part of the claimed design.