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;
FIG. 7 is a bottom view thereof;
FIG. 8 is a perspective view thereof with the lid in an open position; and,
FIG. 9 is an isolated view of the foot of the cooler, shown separately for clarity of illustration.
The features presented in broken lines, including the threaded foot of the cooler, are for environmental purposes only and form no part of the claimed design.