The ornamental design for a cooler, as shown and described.
FIG. 1 shows a right, front perspective view of a cooler showing my new design.
FIG. 2 shows a top view of the cooler of FIG. 1.
FIG. 3 shows a front view of the cooler of FIG. 1.
FIG. 4 shows a back view of the cooler of FIG. 1.
FIG. 5 shows a bottom view of the cooler of FIG. 1.
FIG. 6 shows a right side view of the cooler of FIG. 1; and,
FIG. 7 shows a left side view of the cooler of FIG. 1.
The shorter dashed broken lines in FIGS. 1-7 are for illustrative purposes only and form no part of the claimed design. The long-dash short-dash long-dash broken lines shown in FIGS. 1-5 define the boundary of the claim, which extends to the boundary but does not include the boundary.