8256370: Add asserts to Reference.getInactive()

Reviewed-by: mchung

src/java.base/share/classes/java/lang/ref/Finalizer.java

@@ -83,7 +83,8 @@ final class Finalizer extends FinalReference<Object> { /* Package-private; must
 
         try {
             Object finalizee = this.getInactive();
-            if (finalizee != null && !(finalizee instanceof java.lang.Enum)) {
+            assert finalizee != null;
+            if (!(finalizee instanceof java.lang.Enum)) {
                 jla.invokeFinalize(finalizee);
 
                 // Clear stack slot containing this variable, to decrease

src/java.base/share/classes/java/lang/ref/Reference.java

@@ -353,6 +353,8 @@ public abstract class Reference<T> {
      * null, and would subsequently not finalize the referent/finalizee.
      */
     T getInactive() {
+        assert this instanceof FinalReference;
+        assert next == this; // I.e. FinalReference is inactive
         return this.referent;
     }