nbd: fix -Werror=maybe-uninitialized false-positive

../nbd/client-connection.c:419:8: error: ‘wait_co’ may be used uninitialized [-Werror=maybe-uninitialized]

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

nbd/client-connection.c

@@ -410,7 +410,7 @@ nbd_co_establish_connection(NBDClientConnection *conn, NBDExportInfo *info,
  */
 void nbd_co_establish_connection_cancel(NBDClientConnection *conn)
 {
-    Coroutine *wait_co;
+    Coroutine *wait_co = NULL;
 
     WITH_QEMU_LOCK_GUARD(&conn->mutex) {
         wait_co = g_steal_pointer(&conn->wait_co);