The ornamental design for a safe, as shown and described.
FIG. 1 is a perspective view of a safe showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is another perspective view thereof.
The broken lines shown in the drawings represent portions of the safe that form no part of the claimed design. The interior of the bottom surface forms no part of the claimed design.