The ornamental design for a lamp, as shown and described.
FIG. 1 is a front and bottom perspective view of a lamp, showing my new design;
FIG. 2 is a rear and top perspective view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof;
FIG. 7 is a front elevation view thereof;
FIG. 8 is a rear elevation view thereof; and,
FIG. 9 is an enlarged fragmentary view taken from the rectangle 9 in FIG. 2.
The dashed broken lines in the figures illustrate portions of the article that form no part of the claimed design. The dash dot dash lines in FIGS. 2 and 9 are for the propose of depicting the cut line of the enlarged view and form no part of the claim.