I claim the ornamental design for a buffer strip, as shown and described.
FIG. 1 is a front view of a buffer strip;
FIG. 2 is a top view thereof front view thereof;
FIG. 3 is a back side view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is an isometric view of two buffer strips ready to be interlocked, by movement perpendicular to the front view, into an interlocked buffer strip which may be of a different length.