The ornamental design for a zig-zag zipper, as shown and described.
FIG. 1 is a top plan view of a zig-zag zipper showing my new design;
FIG. 2 is a bottom plan view thereof;
FIG. 3 is a right side elevational view thereof;
FIG. 4 is a left side elevational view thereof, with FIG. 4 being a mirror image of FIG. 3;
FIG. 5 is a front side elevational view thereof;
FIG. 6 is a back side elevational view thereof; and,
FIG. 7 is a perspective view as seen from the top.
The broken lines in FIG. 7 showing the body of a jacket using the zig-zag zipper are used for illustrative purposes only and from no part of the claimed design.