The ornamental design for a cracker, as shown and described.
FIG. 1 is a top view of a cracker showing my new design, according to a first embodiment;
FIG. 2 is a right side view of the design shown in FIG. 1, wherein the left side view is a mirror image thereof;
FIG. 3 is a front view of the design shown in FIG. 1, wherein the rear view is identical thereto; and,
FIG. 4 is a top view of a cracker showing my new design, according to a second embodiment, wherein the right and left side views are identical to the design shown in FIG. 2, and wherein the front and rear views are identical to the design shown in FIG. 3.