[evolution] Avoid a (rather rare) crash under message-list.c::build_tree()

[Milan Crha <mcrha src gnome org>]

commit e9a1508ce1eb5bab2c31498d91104414637ce609
Author: Milan Crha <mcrha redhat com>
Date:   Fri Jan 13 16:04:48 2017 +0100

    Avoid a (rather rare) crash under message-list.c::build_tree()
    
    I hit it when the CamelFolder returned something unexpected, then
    the variable as NULL and it caused the crash.

 src/mail/message-list.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/mail/message-list.c b/src/mail/message-list.c
index c7fe249..070f295 100644
--- a/src/mail/message-list.c
+++ b/src/mail/message-list.c
@@ -4229,7 +4229,7 @@ build_tree (MessageList *message_list,
        build_subtree (
                message_list,
                message_list->priv->tree_model_root,
-               thread->tree, &row);
+               thread ? thread->tree : NULL, &row);
 
        message_list_tree_model_thaw (message_list);