(mbox_append_message0): Preserve return status from mu_stream_write, if it is not 0.

@@ -1721,10 +1721,10 @@ mbox_append_message0 (mu_mailbox_t mailbox, mu_message_t msg, mu_off_t *psize,
 		*psize += n;
 	      }
 	    while (nread > 0);
-	    n = 0;
-       	    status = mu_stream_write (mailbox->stream, &nl, 1, *psize, &n);
 	    if (status)
 	      break;
+       	    status = mu_stream_write (mailbox->stream, &nl, 1, *psize, &n);
+	    if (status == 0)
 	      *psize += n;
 	  }