The ornamental design for a bag, as shown and described.
FIG. 1 is a front perspective view of a bag showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is an opposite view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The even-length broken lines within the shaded area represent stitching. The even-length broken lines depicting the remainder of the bag show features that form no part of the claimed design.