Clean up a clangalyzer warning in directory_remove_invalid

"At this point in the code, msg has been set to a string
constant. But the tor code checks that msg is not NULL, and the
redundant NULL check confuses the analyser[...] To avoid this
spurious warning, the patch initialises msg to NULL."

Patch from teor. another part of 13157.

src/or/dirserv.c

@@ -664,7 +664,7 @@ directory_remove_invalid(void)
   smartlist_add_all(nodes, nodelist_get_list());
 
   SMARTLIST_FOREACH_BEGIN(nodes, node_t *, node) {
-    const char *msg;
+    const char *msg = NULL;
     routerinfo_t *ent = node->ri;
     char description[NODE_DESC_BUF_LEN];
     uint32_t r;