The ornamental design for a pillow, as shown and described.
FIG. 1 is a top plan view of the present invention, showing the pillow of the new design;
FIG. 2 is a bottom plan view thereof;
FIG. 3 is a front side elevational view thereof; and,
FIG. 4 is a back side elevational view thereof.
The broken lines shown in the drawings FIGS. 1-4 are included for the purpose of illustrating portions of the pillow that form no part of the claimed design. The dash-dot line appearing in drawing FIGS. 1-4 represents a boundary line between claimed and unclaimed subject matter and forms no part of the claimed design.