The ornamental design for a lock, as shown and described.
FIG. 1 is a perspective view of a lock showing my new design;
FIG. 2 is a front view of the lock thereof;
FIG. 3 is a rear view of the lock thereof;
FIG. 4 is a right view of the lock thereof;
FIG. 5 is a left view of the lock thereof;
FIG. 6 is a top view of the lock thereof; and,
FIG. 7 is a bottom view of the lock thereof.
The shading in the Figures show the character or contour of the surfaces represented but not surface ornamentations. The broken lines in the Figures form no part of the claimed design.