[PATCH] nbd: Prevent NULL pointer dereference in nbd_blockdev_client_closed()

[Alexander Ivanov <alexander.ivanov@virtuozzo.com>]

In some cases, the NBD server can be stopped before
nbd_blockdev_client_closed() is called, causing the nbd_server variable
to be nullified. This leads to a NULL pointer dereference when accessing
nbd_server.

Add a NULL check for nbd_server to the nbd_blockdev_client_closed()
function to prevent NULL pointer dereference.

Signed-off-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
---
 blockdev-nbd.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/blockdev-nbd.c b/blockdev-nbd.c
index 213012435f..fb1f30ae0d 100644
--- a/blockdev-nbd.c
+++ b/blockdev-nbd.c
@@ -52,6 +52,9 @@ int nbd_server_max_connections(void)
 static void nbd_blockdev_client_closed(NBDClient *client, bool ignored)
 {
     nbd_client_put(client);
+    if (nbd_server == NULL) {
+        return;
+    }
     assert(nbd_server->connections > 0);
     nbd_server->connections--;
     nbd_update_server_watch(nbd_server);
-- 
2.43.0