fix segfault due to missing LinkTable allocation

LinkTable_new dereferences a NULL linktbl pointer when it does not load
the link table from disk. It needs to allocate the new LinkTable object.

Fixes: 0747566 ("updated LinkTable invalidation")

src/link.c

@@ -607,6 +607,7 @@ LinkTable *LinkTable_new(const char *url)
      * disk
      */
     if (!linktbl) {
+        linktbl = LinkTable_alloc(url);
         linktbl->index_time = time(NULL);
         lprintf(debug, "linktbl->index_time: %d\n", linktbl->index_time);