The GVariant constructor g_variant_new () gives a floating reference,
which will be sunk by the g_dbus_..._call () methods. Unless the
constructed GVariant is manually referenced, it should not be used in a
g_autoptr () setting. Otherwise a use-after-free might occur in the
async g_dbus_..._call () methods.
This problem occurs here, which is simply prevented by not having the
g_autoptr () declaration for the GVariant.
Fixes#2647Fixes#2704Fixes#2635Fixes#2706
