The ornamental design for a bag, as shown and described.
FIG. 1 is a top, left and front perspective view of the bag showing my new design, wherein the front flap of the bag is pulled down;
FIG. 2 is a front elevation view;
FIG. 3 is a rear elevation view;
FIG. 4 is a left side elevation view;
FIG. 5 is a right side elevation view;
FIG. 6 is a top plan view;
FIG. 7 is a bottom plan view; and,
FIG. 8 is a top, left and front perspective view with the front flap pulled up.
The broken line showing of portions of the bag is included for the purpose of illustrating environmental structure and forms no part of the claimed design.