Don't return NULL if there's no error message;

return a generic text message instead.
(Someday, I'll track down all the places that
are generating errors but not recording messages. ;-/

Thanks to: Jaakko Heinonen

lib/libarchive/archive_util.c

@@ -65,7 +65,7 @@ archive_error_string(struct archive *a)
 	if (a->error != NULL  &&  *a->error != '\0')
 		return (a->error);
 	else
-		return (NULL);
+		return ("(Empty error message)");
 }