ag_account_set_value(): don't leak the temporary GVariant
ag_account_set_value(): don't leak the temporary GVariant

GVariants created with g_dbus_gvalue_to_gvariant() are never floating,
so need to be unreferenced after being used.

plotters/accounts-sso.libaccounts-glib,plotters/accounts-sso.libaccounts-glib,plotters/accounts-sso.libaccounts-glib