The ornamental design for the nipper sharpener, as shown and described.
FIG. 1 is a front, top, and right-side perspective view of the nipper sharpener showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a right side elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a bottom, rear, and right-side perspective view thereof.
The broken lines showing openings in FIGS. 1 & 2 are included for the purpose of showing environmental structure and form no part of the claimed design.