The ornamental design for a bag, as shown and described.
1. Bag
1.1: Perspective
1.2: Front
1.3: Back
1.4: Left
1.5: Right
1.6: Top
1.7: Bottom
In the representations of the design, broken lines on the outside surfaces of the bag represent stitching and form part of the claimed design; broken lines on the inside of the bag as seen in FIGs. 1.1and 1.6represent portions of the bag that form no part of the claimed design.