The ornamental design for a pacifier, as shown and described.
FIG. 1 is a rear perspective view of the pacifier showing my design;
FIG. 2 is a rear view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is a left side view thereof.
The pacifier handle is shown with a symbolic break near its distal end. The appearance of any portion of the article between the break lines forms no part of the claimed design.