The ornamental design for a lamp, as shown and described.
FIG. 1 is a front, right, and top perspective view of a lamp, showing my design.
FIG. 2 is a front elevation view thereof.
FIG. 3 is a rear elevation view thereof.
FIG. 4 is a left side elevation view thereof.
FIG. 5 is a right side elevation view thereof.
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line showing in the drawings is included for the purpose of illustrating unclaimed portions of the lamp and forms no part of the claimed design.