The ornamental design for a lock for a handbag, as shown and described.
FIG. 1 is a front perspective view of a lock for a handbag showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a left view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top side view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a reduced scale, environmental perspective view thereof.
The short dotted lines illustrate portions of the handbag and form no part of the claimed design. Portions of the clasp not visible in the Figures form no part of the claim.