The ornamental design for a bag, as shown and described.
1. Bag
1.1: Perspective
1.2: Front
1.3: Back
1.4: Right
1.5: Left
1.6: Top
1.7: Bottom
1.8Perspective with shoulder strap shown in an alternate position
1.9Perspective with shoulder strap shown in another alternate position
1.10Perspective with open flap and shoulder strap shown in the position of 1.8
The bag of the design is shown in greyscale, as protection of the design is not to be limited to any specific colour and/or combination of colours of the bag and of portions thereof.