Fix NPE when reparenting a leash that has already been released.

TIL that even when reparenting to null, the leash has to be valid (i.e.
have an underlying native object associated to it). This is only true
when the leash has not yet been released.

Fix: 404507270
Flag: EXEMPT bugfix
Test: manual
Change-Id: I9fa182cec821e838bfe1ef7769f1a7405291019e