Fix unappropriated log of unknown storage class

A warn log may appear when rebuilding the history file from a tradspool
directory whereas there is nothing to worry about (as SMerrorstr is not
set for the corresponding article wrongly reported in error).

Thanks to Jesse Rehmer for the bug report.

storage/tradspool/tradspool.c

@@ -1277,7 +1277,7 @@ tradspool_next(ARTHANDLE *article, const RETRTYPE amount)
         /* Only log an error if art->len is non-zero, since otherwise we get
            all the ones skipped via the hard-link skipping algorithm
            commented above. */
-        if (art->len > 0)
+        if (art->len > 0 && sub == NULL)
             warn("tradspool: can't determine class of %s: %s",
                  TokenToText(token), SMerrorstr);
     } else {