I claim the ornamental design for an adapter for a motorcycle steering stabilizer, as shown.
FIG. 1 is a perspective view from slightly above the adapter for a motorcycle steering stabilizer.
FIG. 2 is a back elevational view thereof.
FIG. 3 is a front elevational view thereof.
FIG. 4 is a top plan view thereof.
FIG. 5 is a bottom plan view thereof.
FIG. 6 is an elevational view of one side thereof; and,
FIG. 7 is an elevational view of the opposite side thereof.
The broken lines showing the triple clamp illustrate environmental structure that forms no part of the claimed design. The broken lines shown in the shape of a square in the center of FIG. 5 represent boundaries between claimed and unclaimed portions of the adapter for a motorcycle steering stabilizer; the broken line boundaries form no part of the claimed design.