I claim, the ornamental design for a base for a desk lamp, as shown and described.
FIG. 1 is a perspective view of a base for a desk lamp;
FIG. 2 is an enlarged perspective view of the base for a desk lamp;
FIG. 3 is an enlarged portion of the perspective view of FIG. 2 of the base for a desk lamp; the broken lines indicate a boundary of the drawing and form no part of the claimed design;
FIG. 4 is a top view of the base for a desk lamp;
FIG. 5 is a bottom view of the base for a desk lamp;
FIG. 6 is a right side view of the base for a desk lamp;
FIG. 7 is a left side view of the base for a desk lamp;
FIG. 8 is a front view of the base for a desk lamp; and,
FIG. 9 is a rear view of the base for a desk lamp.
The broken lines in the drawings illustrates the environment of the base for a desk lamp which forms no part of the claimed design.