commit 2e668a61d5ae4("Fix error handling when adding MCS labels") uses
the 'pctx' in virReportError after it has been freed. Fix it.
Fixes: 2e668a61d5ae4cbd6f79e096d0c394f186e132bd
Signed-off-by: Zhenyu Ye <yezhenyu2@huawei.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>