The ornamental design for a pillow, as shown and described.
FIG. 1 is a perspective view of a pillow;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines immediately adjacent the solid line in FIG. 3 represent the bounds of the claimed design while all other broken lines are for illustrative purposes only; the broken lines form no part of the claimed design.