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 left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a rear right perspective view thereof.
The strap element is shown with a symbolic break in its length. The appearance of any portion of the article between break lines forms no part of the claimed design.