Leads can be re-attached to a dolphin, leaving the original lead knot behind

Discovered while testing: MC-217677.

The bug

As I was testing MC-217677, I noticed that I could re-attach the dolphin to a new lead, which would remove the original lead. However, the knot from the original lead would remain there. I was in creative mode, not sure if that matters.

Steps to reproduce

1. Connect a dolphin to a lead.
2. Connect the lead to a fence.
3. Connect the dolphin to a new lead.  There will be a glitched lead knot with no lead attached.