I claim the ornamental design for a diaper bag, as shown and described.
FIG. 1 is a Perspective view of a diaper bag showing my new design;
FIG. 2 is a Front view thereof;
FIG. 3 is a Back view thereof;
FIG. 4 is a Right-side view thereof;
FIG. 5 is a Left-side view thereof;
FIG. 6 is a Top view thereof; and,
FIG. 7 is a Bottom view thereof.
The broken lines on the surface of the diaper bag depict stitching and form no part of the claim. The broken lines depicting zipper tread and zipper toggles depict portions of the article and form no part of the claim.