Refactor locking in setSuperclass.

Now all lock ownership is tied to scopes, so will be released correctly
in all return paths.

runtime.c

@@ -522,19 +522,19 @@ Class class_setSuperclass(Class cls, Class newSuper)
 {
 	CHECK_ARG(cls);
 	CHECK_ARG(newSuper);
+	Class oldSuper;
 	if (Nil == cls) { return Nil; }
 
-	LOCK_RUNTIME();
+	{
+		LOCK_RUNTIME_FOR_SCOPE();
 
-	if (cls->super_class == newSuper) {
+		oldSuper = cls->super_class;
-		UNLOCK_RUNTIME();
+
-		return newSuper;
+		if (oldSuper == newSuper) { return newSuper; }
-	}
 
 		safe_remove_from_subclass_list(cls);
 		objc_resolve_class(newSuper);
 
-	Class oldSuper = cls->super_class;
 		cls->super_class = newSuper;
 
 		// The super class's subclass list is used in certain method resolution scenarios.
@@ -561,18 +561,14 @@ Class class_setSuperclass(Class cls, Class newSuper)
 			newSuperMeta->subclass_list = meta;
 		}
 
-	LOCK(&initialize_lock);
+		LOCK_FOR_SCOPE(&initialize_lock);
 		if (!objc_test_class_flag(cls, objc_class_flag_initialized))
 		{
 			// Uninitialized classes don't have dtables to update
 			// and don't need their superclasses initialized.
-		UNLOCK(&initialize_lock);
-		UNLOCK_RUNTIME();
 			return oldSuper;
 		}
-
+	}
-	UNLOCK(&initialize_lock);
-	UNLOCK_RUNTIME();
 
 	objc_send_initialize((id)newSuper); // also initializes the metaclass
 	objc_update_dtable_for_new_superclass(cls->isa, newSuper->isa);