The ornamental design for an adapter, as shown and described.
FIG. 1 is a perspective view of an adapter showing my new design;
FIG. 2 is another perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines depict portions of the adapter and form no part of the claimed design.
The adapter is shown with a symbolic break in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.