lib: add _notmuch_message_list_empty

There is no public notmuch_message_list_t public interface, so to this
is added to the private API. We use it immediately in thread.cc;
future commits will use it further.

lib/messages.c

@@ -56,6 +56,15 @@ _notmuch_message_list_add_message (notmuch_message_list_t *list,
     list->tail = &node->next;
 }
 
+bool
+_notmuch_message_list_empty (notmuch_message_list_t *list)
+{
+    if (list == NULL)
+	return TRUE;
+
+    return (list->head == NULL);
+}
+
 notmuch_messages_t *
 _notmuch_messages_create (notmuch_message_list_t *list)
 {

lib/notmuch-private.h

@@ -477,6 +477,9 @@ struct _notmuch_messages {
 notmuch_message_list_t *
 _notmuch_message_list_create (const void *ctx);
 
+bool
+_notmuch_message_list_empty (notmuch_message_list_t *list);
+
 void
 _notmuch_message_list_add_message (notmuch_message_list_t *list,
 				   notmuch_message_t *message);

lib/thread.cc

@@ -425,7 +425,7 @@ _resolve_thread_relationships (notmuch_thread_t *thread)
     if (first_node) {
 	message = first_node->message;
 	in_reply_to = _notmuch_message_get_in_reply_to (message);
-	if (thread->toplevel_list->head &&
+	if (! _notmuch_message_list_empty (thread->toplevel_list) &&
 	    in_reply_to && strlen (in_reply_to) &&
 	    g_hash_table_lookup_extended (thread->message_hash,
 					  in_reply_to, NULL,