I claim the ornamental design for a lap scarf, as shown and described.
FIG. 1 is a perspective view of a lap scarf, showing my new design;
FIG. 2 is a top plan view thereof, the bottom plan view being a mirror image thereof; and,
FIG. 3 is a front elevational view thereof, the rear elevational view, left-side elevatinoal view, and right-side elevatinoal view all being identical thereto.