The ornamental design for a cooler, as shown and described.
FIG. 1 is an upper front right perspective view of a cooler showing our new design;
FIG. 2 is a front perspective view thereof;
FIG. 3 is a rear perspective view thereof;
FIG. 4 is a right side perspective view thereof;
FIG. 5 is a left side perspective view thereof; and,
FIG. 6 is a top perspective view thereof.
The “RED BULL” name and symbol shown in FIGS. 1-6, as well as the “ENERGY DRINK” indicia shown in FIGS. 1 and 2 and the labels shown in FIG. 3 are shown in broken line for purposes of illustrating environment and form no part of the claimed design.