The ornamental design for a bag, as shown and described.
1. Bag
1.1: Front
1.2: Unfolded
1.3: Back
1.4: Unfolded
1.5: Top
1.6: Unfolded
1.7: Bottom
1.8: Unfolded
1.9: Left
1.10: Unfolded
1.11: Right
1.12: Unfolded
1.13: Perspective
1.14: Unfolded
The broken lines in the figures are for the purpose of illustrating claimed stitching; 1.1is a front view of a bag; 1.2is an unfolded front view thereof; 1.3is a back view thereof; 1.4is an unfolded back view thereof; 1.5is a top view thereof; 1.6is an unfolded top view thereof; 1.7is a bottom view thereof; 1.8is an unfolded bottom view thereof; 1.9is a left view thereof; 1.10is an unfolded left view thereof; 1.11is a right view thereof; 1.12is an unfolded right view thereof; 1.13is a perspective view thereof; 1.14is an unfolded perspective view thereof.