The ornamental design for a scarf, as shown and described.
FIG. 1 is a front perspective view of my new scarf design, the rear perspective view being a mirror image thereof;
FIG. 2 is a right side elevation view of the scarf, the left side view being a mirror image thereof;
FIG. 3 is a front elevation view shown in an environment of use thereof; and,
FIG. 4 is another front elevation view shown in an environment of use thereof.
Any broken line illustration of environmental structure in the drawings is not part of the design sought to be patented. The bracketed separations shown in FIG. 1 indicate that any portion of the article beyond what is shown forms no part of the claimed design.