Make Realm::is_closed() work on read-only instances

src/shared_realm.hpp

@@ -132,7 +132,7 @@ namespace realm {
         // Realm after closing it will produce undefined behavior.
         void close();
 
-        bool is_closed() { return m_shared_group == nullptr; }
+        bool is_closed() { return !m_read_only_group && !m_shared_group; }
 
         ~Realm();