I claim the ornamental design for a sun lounger, as shown and described.
FIG. 1 is a right side view of the sun lounger showing my new design, the left side view being a mirror image of the right side view;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a front perspective view thereof;
FIG. 6 is a rear perspective view thereof;
FIG. 7 is an underside view thereof; and,
FIG. 8 is a right side view of the sun lounger showing a foot rest, a back rest and a sunshade of the sun lounger in different positions than the position of the foot rest, the back rest and the sunshade shown in the other Figures.