hw/net/xen_nic.c: Set 'netdev->mac' to NULL after free it

Since net_init() checks whether 'netdev->mac' is NULL, before alloc it;
net_release() also need set 'netdev->mac' to NULL after free it.
Signed-off-by: NChen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/net/xen_nic.c

@@ -428,6 +428,7 @@ static int net_free(struct XenDevice *xendev)
         netdev->nic = NULL;
     }
     g_free(netdev->mac);
+    netdev->mac = NULL;
     return 0;
 }