The ornamental design for an adapter, as shown and described.
FIG. 1 is a front elevational view of the adapter showing my new design; and,
FIG. 2 is an enlarged front elevational view of the new design for the purpose of showing small details of the new design.
The dashed line showing of the environmental structure in FIGS. 1-2 represents unclaimed detail only and forms no part of the claimed design. The dashed-dotted lines show a boundary between the claimed design and the environmental structure and form no part of the claimed design.