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 elevational view thereof;
FIG. 3 is a rear elevational 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;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is an assembled view for showing a shackle is assembled to the lock, wherein the broken line portion is for illustrative purpose only and forms no part of the claimed design.