We had documented both notmuch_thread_results_get and
notmuch_message_results_get to return NULL if (! has_more)
but we hadn't actually implemented that. Fix.
     Xapian::docid doc_id;
     notmuch_private_status_t status;
 
+    if (! notmuch_message_results_has_more (results))
+       return NULL;
+
     doc_id = *results->iterator;
 
     message = _notmuch_message_create (results,
     notmuch_thread_t *thread;
     const char *thread_id;
 
+    if (! notmuch_thread_results_has_more (results))
+       return NULL;
+
     thread_id = (const char *) g_ptr_array_index (results->thread_ids,
                                                  results->index);