We claim the ornamental design for an lamp, as shown and described.
FIG. 1 is a front view of a lamp of the present invention;
FIG. 2 is a rear view thereof;
FIG. 3 is a first side view thereof;
FIG. 4 is a second side view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken line disclosure of the lamp base and components within the outer envelope is for illustrative purposes only and forms no part of the claimed design.