Fix instance binding unreference callback regression

This was a regression from 4469144891

The callback should be called, not only be called when the refcount
reaches 0. For example, the C# callback needs to know when the
refcount reaches 1, in order to swap to a weak GC handle.

core/object/ref_counted.cpp

@@ -85,7 +85,8 @@ bool RefCounted::unreference() {
 			_get_extension()->unreference(_get_extension_instance());
 		}
 
-		die = die && _instance_binding_reference(false);
+		bool binding_ret = _instance_binding_reference(false);
+		die = die && binding_ret;
 	}
 
 	return die;