The ornamental design for a lamp, as shown and described.
FIG. 1 is a top, rear, and right side perspective 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 left side elevational view thereof;
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 disclose a portion of the article in which the claimed design is embodied, but which forms no part of the claimed design.