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 my new design; and,
FIG. 2 is a front elevational view thereof.
The broken lines shown in FIGS. 1 and 2 illustrate portions of the lamp that form no part of the claimed design. The boundary lines, shown in alternating long and short broken line segments, form no part of the claimed design.