We claim the ornamental design for the swaddle sack, as shown and described.
FIG. 1 is a perspective view of the swaddle sack, where features illustrated in broken lines show environmental aspects and form no part of the claimed design;
FIG. 2 is a front elevation view of the swaddle sack with foldover mitten cuffs in an open position;
FIG. 3 is a right side elevation view of the swaddle sack with foldover mitten cuffs in the open position, the left side view being a mirror image and not shown;
FIG. 4 is a rear elevation view of the swaddle sack with foldover mitten cuffs in the open position;
FIG. 5 is a front elevation view of the swaddle sack with foldover mitten cuffs in a closed position;
FIG. 6 is a right side elevation view of the swaddle sack with foldover mitten cuffs in the closed position, the left side view being a mirror image and not shown; and,
FIG. 7 is a rear elevation view of the swaddle sack with foldover mitten cuffs in the closed position.