The ornamental design for a bag, as shown and described.
FIG. 1 is a front view of a bag showing my new design;
FIG. 2 is a rear view of the bag shown in FIG. 1;
FIG. 3 is a left side view of the bag shown in FIG. 1;
FIG. 4 is a right side view of the bag shown in FIG. 1;
FIG. 5 is a bottom view of the bag shown in FIG. 1; and,
FIG. 6 is a perspective view of the bag shown in FIG. 1.
The broken lines showing the handbag strap, fasteners and stitching are for the purposes of illustrating environment and form no part of the claimed design.