The ornamental design for a stairstepmat, as shown and described.
The following article relates to a small carpet mat adapted to be installed on stair steps, separately on each step, especially stairs located inside the house.
FIG. 1 is a top front perspective view of a stair step mat showing my new design;
FIG. 2 is a left side elevational view of the stair step mat, the left side being identical thereto;
FIG. 3 is a front elevational view of the front side of the stair step mat, the backside being identical thereto;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view thereof.
The broken lines in the drawings depict environmental subject matter, which forms no part of the claimed design.