The ornamental design for a bag, as shown and described.
FIG. 1 is a top front perspective view of a bag showing the claimed design in an open configuration;
FIG. 2 is a front view thereof in a closed configuration;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof, the right side view being a mirror image thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The dashed broken lines in the figures show environment, which forms no part of the claimed design.
The dot-dash broken lines indicating a separation and the portions between the separations form no part of the claimed design, and are used to indicate that no particular length for the bag between the separations is claimed.