xen/netfront: fix crash when removing device

When removing a netfront device directly after a suspend/resume cycle
it might happen that the queues have not been setup again, causing a
crash during the attempt to stop the queues another time.

Fix that by checking the queues are existing before trying to stop
them.

This is XSA-465 / CVE-2024-53240.

Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Fixes: d50b7914 ("xen-netfront: Fix NULL sring after live migration")
Signed-off-by: Juergen Gross <jgross@suse.com>