linke_message: Avoid segfault when In-Reply-to header is empty.

This was recently introduced in commit:

	64c03ae97f2f5294c60ef25d7f41849864e6ebd3

which was adding extra checks to avoid adding a self-referencing
message.

How many times am I going to fix a dumb regression like this and say
"we really need a test suite" before I actually sit down and write the
test suite?

diff --git a/lib/database.cc b/lib/database.cc
index 65478b8a30c4ec217c293c95675d0090afa25dba..ce9a86bfddb8b525a19ecefe27d443bb803a8d01 100644 (file)
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -726,7 +726,8 @@ _notmuch_database_link_message_to_parents (notmuch_database_t *notmuch,
 
     /* Carefully avoid adding any self-referential in-reply-to term. */
     in_reply_to_message_id = _parse_message_id (message, in_reply_to, NULL);
-    if (strcmp (in_reply_to_message_id,
+    if (in_reply_to_message_id &&
+       strcmp (in_reply_to_message_id,
                notmuch_message_get_message_id (message)))
     {
        _notmuch_message_add_term (message, "replyto",