The ornamental design for a bag, as shown and described.
FIG. 1 is a Top, Left and Front perspective view with the top of the front pocket pulled slightly away from the front surface of the bag;
FIG. 2 is a Front elevation view with the top of the front pocket pressed against the front surface of the bag;
FIG. 3 is a Rear elevation view;
FIG. 4 is a Right side elevation view;
FIG. 5 is a Left side elevation view;
FIG. 6 is a Top plan view; and,
FIG. 7 is a Bottom plan view.
The broken line showing the zipper across the top and the handles is included for the purpose of illustrating portions of the article and forms no part of the claimed design.