The ornamental design for a strap for bag, as shown and described.
FIG. 1 is a front view of a strap for bag showing my new design;
FIG. 2 is a left side view thereof (the right side view being a mirror image thereof); and,
FIG. 3 is a top view thereof.
The middle of the strap for bag is shown with symbolic breaks in its length. The appearance of any portion of the article between the two break lines forms no part of the claimed design. The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design while all other broken lines are directed to environment; the broken lines form no part of the claimed design.