The ornamental design for a buckle for bag, as shown and described.
FIG. 1 is a front, top and right perspective view of a buckle for bag showing claimed design;
FIG. 2 is a front view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof;
FIG. 7 is an exploded view thereof with a strap attached thereto;
FIG. 8 is another exploded view thereof with a portion thereof open; and,
FIG. 9 is a reference view showing the buckle for bag in use.
The portions of the buckle for bag shown in broken lines in FIGS. 3, 7, and 8 form no part of the claimed design. Environment shown in broken lines in FIGS. 7-9 form no part of the claimed design.