We claim the ornamental design for a locking device, as shown and described in FIGS. 1-8.
FIG. 1 is a front, right pictorial view of a locking device according to another embodiment;
FIG. 2 is a rear, left pictorial view of the locking device of FIG. 1;
FIG. 3 is a front elevational view of the locking device of FIG. 1;
FIG. 4 is a rear elevation view of the locking device of FIG. 1;
FIG. 5 is a right side elevation view of the locking device of FIG. 1;
FIG. 6 is a left side elevation view of the locking device of FIG. 1;
FIG. 7 is a top plan view of the locking device of FIG. 1; and,
FIG. 8 is a bottom plan view of the locking device of FIG. 1.
The broken lines shown in the locking device are for portions that form no part of the claimed design. Additionally, the broken line representations of environment form no part of the claimed design.