The ornamental design for a lamp, as shown and described.
FIG. 1 is a perspective view of the lamp according to the present invention illustrated without a lamp shade and with elements in dashed lines forming no part of the claimed design;
FIG. 2 is another perspective view of the lamp thereof with a lamp shade in place and with elements in dashed lines forming no part of the claimed design;
FIG. 3 is a front view of the lamp thereof with elements in dashed lines forming no part of the claimed design;
FIG. 4 is a back view of the lamp thereof with elements in dashed lines forming no part of the claimed design;
FIG. 5 is a left view of the lamp thereof with elements in dashed lines forming no part of the claimed design;
FIG. 6 is a right view of the lamp thereof with elements in dashed lines forming no part of the claimed design;
FIG. 7 is a top view of the lamp thereof with elements in dashed lines forming no part of the claimed design; and,
FIG. 8 is a bottom view of the lamp thereof with elements in dashed lines forming no part of the claimed design.