The ornamental design for a buffer, as shown and described.
FIG. 1 is a perspective view of a buffer showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a front right perspective view of the buffer in a state of elastic deformation in a use mode which accommodates a digital X-ray sensitive device.
The broken lines shown in the drawings form no part of the claimed design.