The ornamental design for a pillow, as shown and described.
FIG. 1 is a perspective view of the pillow showing the new design in a first state in which the pillow is filled;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view of the pillow in a second state in which the pillow is unfilled; and,
FIG. 7 is a perspective view of the pillow from FIG. 6.
The broken lines immediately adjacent the shaded surfaces represent the bounds of the claimed design. The unshaded surfaces bounded by broken lines form no part of the claimed design. All other broken lines depict subject matter that forms no part of the claimed design.