The ornamental design for combination lock, as shown and described.
FIG. 1 is a front elevational view of a combination lock showing my new design;
FIG. 2 is a rear elevational view;
FIG. 3 is a left side elevational view;
FIG. 4 is a right side elevational view;
FIG. 5 is a top plan view;
FIG. 6 is a bottom plan view;
FIG. 7 is a perspective view of the combination lock; and,
FIG. 8 is a perspective view of the combination lock having a fastening cable secured therewith.
The broken line disclosure of the fastening cable is shown for illustrative purposes only and forms no part of the claimed design.