The ornamental design for a bag, as shown and described.
FIG. 1 is a front, top and side perspective view of a bag showing the new design;
FIG. 2 is a bottom, rear and side perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines immediately adjacent to the claimed portions are boundaries, all other broken lines are environmental structures; all the broken lines form no part of the claimed design.