The ornamental design for a bag, as shown and described.
FIG. 1 is a front left perspective view of a bag showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a top view thereof, showing the shoulder strap in an alternate position.
The portions of the bag shown in even dash broken lines represent thread stitching and do not form part of the claimed design. The portions of bag shown in long dash-dot-long dash broken lines form no part of the claimed design.