The ornamental design for a bag, as shown and described.
FIG. 1 is a front perspective view of a bag showing the new design;
FIG. 2 is a left elevational view thereof;
FIG. 3 is a right elevational view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The dash-dot-dot-dash lines and the break lines through the pull handle illustrate the bounds of the claimed design. The remaining broken lines in the drawings are directed to environment and form no part of the claimed design.