The ornamental design for a bag, as shown and described.
FIG. 1 is a front 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 side view thereof;
FIG. 5 is a second side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines on the surface of the bag represent the bounds of the claimed design. The zipper pulls and other external features shown in broken lines are environmental features that form no part of the claimed design. The broken lines and unshaded regions bounded by broken lines form no part of the claimed design.