The ornamental design for a sandal, as shown and described.
FIG. 1 is a top view of a sandal showing our new design;
FIG. 2 is a front, perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof; and,
FIG. 6 is a right side view thereof.
The broken line showing of an insole, an outsole, and straps is for the purpose of illustrating portions of the sandal and forms no part of the claimed design.