The ornamental design for a sandal, as shown and described.
FIG. 1 is a perspective view of a sandal showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is an enlarged front perspective view thereof;
FIG. 4 is an enlarged rear perspective view thereof;
FIG. 5 is a left side elevation view thereof; and,
FIG. 6 is a right side elevation view thereof.
Broken lines immediately adjacent shaded areas represent unclaimed boundaries of the design. Broken lines showing the remaining of the sandal are for environmental purposes only and form no part of the claimed design.