libnm: fix leak with self assignment in nm_connection_add_setting()

We must consume the reference, like we would in the other case.

Interestingly, I am unable to reproduce a case where valgrind would
complain about the leak. But it is there nonetheless.

Fixes: 0a22f4e4905c ('libnm: refactor tracking of NMSetting in NMConnection')

src/libnm-core-impl/nm-connection.c

@@ -202,8 +202,10 @@ _nm_connection_add_setting(NMConnection *connection, NMSetting *setting)
     priv = NM_CONNECTION_GET_PRIVATE(connection);
 
     s_old = priv->settings[setting_info->meta_type];
-    if (s_old == setting)
+    if (s_old == setting) {
+        g_object_unref(s_old);
         return;
+    }
 
     priv->settings[setting_info->meta_type] = setting;