The ornamental design for a lamp, as shown and described.
FIG. 1 is a front view of a lamp showing my new design, the back side being symmetrical to the front view;
FIG. 2 is a right view of the lamp shown in FIG. 1;
FIG. 3 is a left view of the lamp shown in FIG. 1;
FIG. 4 is a top view of the lamp shown in FIG. 1; and,
FIG. 5 is a bottom view of the lamp shown in FIG. 1.
The structural elements in the figures depicted by means of broken lines have been shown for the purposes of illustrating portions of the lamp which form no part of the claimed design.