ui/spice-display: check NULL pointer in interface_release_resource()

Check rext.info to avoid potential NULL pointer dereference. A similar check
exists in interface_release_resource() in hw/display/qxl.c.

Reported-by: Yu Lu <ini.universe@gmail.com>
Signed-off-by: Mauro Matteo Cascella <mcascell@redhat.com>
Reviewed-by: Prasad J Pandit <pjp@fedoraproject.org>
Message-Id: <20210520105833.183160-1-mcascell@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>