The ornamental design for a cooler, as shown and described.
FIG. 1 is a perspective view of a cooler showing my new design.
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is an opposite side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The features of the claimed design shown in broken lines in the drawings represent environmental structure and portions of the article, and form no part of the claimed design.