Do not unmap uninitialized dma addresses.

Fixes: CVE-2021-3608
Reviewed-by: VictorV (Kunlun Lab) <vv474172261@gmail.com>
Tested-by: VictorV (Kunlun Lab) <vv474172261@gmail.com>
Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Message-Id: <20210630115246.2178219-1-marcel@redhat.com>
Tested-by: Yuval Shaia <yuval.shaia.ml@gmail.com>
Reviewed-by: Yuval Shaia <yuval.shaia.ml@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>