8256040: Shenandoah: Allow NULL referent in ShenandoahReferenceProcessor::should_discover()

Reviewed-by: shade

Author: rkennke

src/hotspot/share/gc/shenandoah/shenandoahReferenceProcessor.cpp

@@ -265,7 +265,7 @@ template <typename T>
 bool ShenandoahReferenceProcessor::should_discover(oop reference, ReferenceType type) const {
   T* referent_addr = (T*) java_lang_ref_Reference::referent_addr_raw(reference);
   T heap_oop = RawAccess<>::oop_load(referent_addr);
-  oop referent = CompressedOops::decode_not_null(heap_oop);
+  oop referent = CompressedOops::decode(heap_oop);
 
   if (is_inactive<T>(reference, referent, type)) {
     log_trace(gc,ref)("Reference inactive: " PTR_FORMAT, p2i(reference));