Don't re-read referent in ReferenceProcessor::GetReferent

Re-reading has the issue that it may read a null value after already
having done the null check. Using a cached value prevents this from
happening and causing DCHECK failures.

Added a related stress test.

Bug: 33569625
Bug: 33389022

Test: test-art-host

Change-Id: Ic42d540e035d41ac6e5b01762f9510cd6632b28c