We claim, the ornamental design for an electrode adapter, as shown and described.
FIG. 1 is a top perspective view of an electrode adapter;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a front elevational view thereof; and,
FIG. 7 is a back elevational view thereof.
The features shown in broken lines in the Figures are for illustrating environmental structure and form no part of the claimed design. The break lines indicate that the appearance of any portion of the electrode adapter between the break lines forms no part of the claimed design.