alive_clients may be cleaned up out of order

Though arguably this shouldn't happen.  For now, just fix the
crash on exit.  We don't need to debug the issue if we're moving
to the new BIO code.

src/lib/io/master.c

@@ -919,14 +919,20 @@ static fr_client_t *radclient_alloc(TALLOC_CTX *ctx, int ipproto, fr_io_address_
  */
 static int _client_live_free(fr_io_client_t *client)
 {
+	talloc_get_type_abort(client, fr_io_client_t);
+
 	fr_assert(client->in_trie);
 	fr_assert(!client->connection);
-	fr_assert(fr_heap_num_elements(client->thread->alive_clients) > 0);
+	fr_assert(client->thread);
 
 	if (client->pending) TALLOC_FREE(client->pending);
 
 	(void) fr_trie_remove_by_key(client->thread->trie, &client->src_ipaddr.addr, client->src_ipaddr.prefix);
-	(void) fr_heap_extract(&client->thread->alive_clients, client);
+
+	if (client->thread->alive_clients) {
+		fr_assert(fr_heap_num_elements(client->thread->alive_clients) > 0);
+		(void) fr_heap_extract(&client->thread->alive_clients, client);
+	}
 
 	return 0;
 }