The ornamental design for a sandal, as shown and described.
1. Sandal
1.1 is a perspective view of a sandal showing a new design;
1.2 is a front view thereof;
1.3 is a rear view thereof;
1.4 is a side view thereof;
1.5 is an opposite side view thereof;
1.6 is a top view thereof; and
1.7 is a bottom view thereof.
The reproductions of design n. 1 show only the right shoe, the left shoe is omitted because it is symmetrical to the right shoe; in the figures the dotted lines represent seams.