NativeObjectManager: clear reference before deleting to avoid a race (#1617)

Author: stephengold

jme3-core/src/main/java/com/jme3/util/NativeObjectManager.java

@@ -140,6 +140,7 @@ private void deleteNativeObject(Object rendererObject, NativeObject obj, NativeO
                 }
 
                 assert ref == null || ref == ref2;
+                ref2.clear();
 
                 int id = obj.getId();