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
1.8 : Perspective
Reproduction 1.8 is a referential perspective view showing the state that a chain is connected to a body of handbag and forms no part of the claimed design.