The ornamental design for a strap for bag, as shown and described.
1. Strap for bag
1.1 : Perspective
1.2 : Top
1.3 : Bottom
1.4 : Front
1.5 : Back
1.6 : Right, with the left view being identical
1.7 : Perspective View
The cutaway symbols illustrated in break lines in the drawings indicate a symbolic break in the length of the design for the strap for bag indicating an indeterminate length. The appearance of any portion of the article between the break lines forms no part of the claimed design.
The broken lines shown in the drawings illustrate portions of the strap for bag that form no part of the claimed design.