The ornamental design for a lamp, as shown and described.
FIG. 1 is a perspective front view of a lamp showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a side elevational view thereof, the opposite side being a mirror image of that shown;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken line disclosure in the views is understood to represent portions of the article in which the claimed design is embodied, but which forms no part of the claimed design.