Make sure to return result from Object::call() in Mixin.

[ci skip]

core/script/mixin_script/mixin_script.cpp

@@ -522,7 +522,7 @@ Variant Mixin::call(const StringName &p_method, const Variant **p_args, int p_ar
 	// The following allows to call `Mixin.free()`.
 	// This is not strictly needed because Mixin objects should not be instantiated directly,
 	// and doing so is discouraged, but this can make debug builds more robust for fuzz testing. 
-	Object::call(p_method, p_args, p_argcount, r_error);
+	return Object::call(p_method, p_args, p_argcount, r_error);
 #endif
 	return Variant();
 }