ivshmem-server: fix possible OVERRUN

>>>     CID 1337991:  Memory - illegal accesses  (OVERRUN)
>>>     Decrementing "i". The value of "i" is now 65534.
218         while (i--) {
219             event_notifier_cleanup(&peer->vectors[i]);
220         }

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
