The ornamental design for a handle for a storage bag, as shown and described.
FIG. 1 is a front perspective view of a handle for a storage bag (the dashed line showing of a storage bag is for environmental purposes and is not considered part of the design);
FIG. 2 is a front view thereof (the rear view is the same as the front view);
FIG. 3 is a top view thereof; and,
FIG. 4 is a right side view thereof (the left side view is the same as the right side).