We claim the ornamental design for a robot cleaner, as shown and described.
FIG. 1 is a front perspective view of a robot cleaner showing our 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; and,
FIG. 7 is a bottom view thereof.
The broken lines in the figures show portions of the robot cleaner which form no part of the claimed design.
Portions of FIG. 5 are illustrated in such a way that the solid lines overlap the broken lines, in order to provide a more complete view of the claimed subject matter.