sd-netlink: make timeout message sealed

When sd_netlink_call_async() timed out, then we reply the synthetic
error message, but it was not sealed. So, reading the message causes
the following assertion:

```
Assertion 'm->sealed' failed at src/libsystemd/sd-netlink/netlink-message.c:652, function netlink_message_read_internal(). Ignoring.
```

src/libsystemd/sd-netlink/netlink-util.c

@@ -236,10 +236,10 @@ int rtnl_message_new_synthetic_error(sd_netlink *rtnl, int error, uint32_t seria
         if (r < 0)
                 return r;
 
+        rtnl_message_seal(*ret);
         (*ret)->hdr->nlmsg_seq = serial;
 
         err = NLMSG_DATA((*ret)->hdr);
-
         err->error = error;
 
         return 0;