I claim the ornamental design for a decorative lamp, as shown and described.
FIG. 1 is a front view of a decorative 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 some parts within the outer envelope is for illustrative purposes only and forms no part of the claimed design.