Assert that the directory server digest is given

This prevents a possible crash when memory is copied from a pointer to
NULL.

src/or/routerlist.c

@@ -4051,6 +4051,8 @@ dir_server_new(int is_authority,
   uint32_t a;
   char *hostname_ = NULL;
 
+  tor_assert(digest);
+
   if (weight < 0)
     return NULL;