The ornamental design for a bag, as shown and described.
FIG. 1 is a perspective view of the bag showing my new design.
FIG. 2 is a front view thereof.
FIG. 3 is a back view thereof.
FIG. 4 is a right view thereof.
FIG. 5 is a left view thereof.
FIG. 6 is a top view thereof.
FIG. 7 is a bottom view thereof.
FIG. 8 is a top view thereof; and,
FIG. 9 is a front view thereof.
The broken lines showing portions of the bag are included for the purpose of illustrating portions of the article and form no part of the claimed design.