The ornamental design for a bag, as shown and described.
FIG. 1 is a front elevational view of an ornamental design for a bag according to the present invention;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a left side elevational view thereof, the opposite side being a mirror image thereof; and,
FIG. 4 is a top view thereof in an open configuration.
The bottom of the bag is unornamented. Throughout the drawings, broken lines show stitching and environment only and form no part of the claimed design.