I claim the ornamental design for a pacifier, as shown and described.
FIG. 1 is a top perspective view of a pacifier showing my new design;
FIG. 2 is a front view thereof, the back view being identical;
FIG. 3 is a left side view thereof, the right side view being identical;
FIG. 4 is a bottom perspective view thereof; and,
FIG. 5 is a bottom plan view thereof.
The broken line showing a stop guard or shield is included for the purpose of illustrating a portion of the article and forms no part of the claimed design.