The ornamental design for a cooler, as shown and described.
FIG. 1 is a bottom, front, left perspective view of a cooler showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a rear view thereof; and,
FIG. 6 is an enlarged perspective view of the claimed portion, taken from FIG. 1.
In the drawing views, the even-broken line represents unclaimed environment of the remainder of the cooler, and form no part of the claimed design. The broken lines that separate the two parts of the cooler and the portion within these broken lines represent indeterminate length and form no part of the claimed design. The dot-dash broken lines represent unclaimed boundaries of the claimed design. The wavy broken lines in FIG. 6 show an enlarged view of the claimed portion in FIG. 1, and form no part of the claimed design.