Cannot leash multiple mobs to same fence when not holding lead item

The bug

Mabye caused by the fix of MC-21433 you are unable to leash multiple mobs to the same fence if you are not holding a lead item. This happened first in 16w39c.

Expected behavior

You can leash like it was in previous versions as much mobs as you want to a fence regardless of the held item.

How to reproduce

1. Place a fence
2. Leash one mob to the fence
3. Try to leash another mob to the fence while not holding a lead item
   → It does not work
4. Try again while holding a lead item
   → It works