I claim the ornamental design for an aisle barrier, as shown and described.
FIG. 1 is a front view.
FIG. 2 is a top view; and,
FIG. 3 is a left side view, and the right side view is a mirror image thereof.
The cross-hatching is shown for illustrative purposes only and form no part of the claimed design.
Portions of the drawings shown in dashed line are also for illustrative purposes only and form no part of the claimed design. The rear view forms no part of the design and is omitted.