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 rear perspective view thereof;
FIG. 3 is a bottom view thereof; and,
FIG. 4 is a partial enlarged view of FIG. 1.
The dotted lines shown within the solid lines in FIGS. 1-4 represent stitching.
The dash-dot broken lines in FIG. 4 enclose portions of the claimed design that are illustrated in enlargements and form no part of the claimed design.