The ornamental design for a lamp, as shown and described.
FIG. 1 is an oblique view of the lamp;
FIG. 2 is a front view thereof, the rear, left side, and right side views being identical thereto;
FIG. 3 is a bottom view thereof; and,
FIG. 4 is a top view thereof.
The dashed line showing of structural lines is included for the purpose of illustrating non-claimed subject matter and forms no part of the claimed design.