The ornamental design for a sanitarynapkin, as shown and described.
FIG. 1 is a bottom plan view of a design for a sanitary napkin, according to the present invention showing the design for the garment-facing surface thereof; and,
FIG. 2 is a top plan view thereof.
Those portions of the drawings depicted in broken lines are provided for environmental purposes only and form no part of the claimed design. Front, back and side views have been omitted since the claimed design is two dimensional.