The ornamental design for the nail buffer, as shown and described.
FIG. 1 is a perspective view of my design for a nail buffer;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a right side view thereof.
The outermost broken line indicates a portion of the article and the innermost broken line indicates the boundary of the claimed design. None of the broken lines forms part of the claimed design.