The ornamental design for a bag, as shown and described.
FIG. 1 is a perspective view of a bag showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof; and,
FIG. 4 is a top plan view thereof.
In the drawing figures, even broken lines on the bag represent stitch lines, while broken lines of a dot-dash pattern adjacent shaded areas denote the bounds of the claim and form no part of the claimed design. The broken line showing of handles and a tassel is for illustrative purposes only and forms no part of the claim.