The ornamental design for a lamp, as shown and described.
FIG. 1 is a perspective view of a lamp showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is an enlarged left view thereof;
FIG. 5 is an enlarged right view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a partially enlarged view of FIG. 2.
The broken-line square, which illustrates the boundary of the enlarged view shown in FIGS. 2 and 8, along with the related broken line in FIG. 2, together represent a detail callout, which form no part of the claimed design.