The ornamental design for a silencer, as shown and described.
FIG. 1 is a perspective view of a silencer showing my new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a front view thereof; and,
FIGS. 4-7 are side views thereof.
The broken lines in the figures are for the purpose of illustrating environmental structure only and form no part of the claimed design. Any surface or portion of our design not shown in the figures or otherwise described herein forms no part of the claimed design.