The ornamental design for a monitor for a safe, as shown and described.
FIG. 1 is a left side, top perspective view of the monitor for a safe showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line showing of monitor for a safe is included for the purpose of illustrating portions of the monitor for a safe that form no part of the claimed design.