log-iter-xml: Fix crash when logs are corrupted or invalid XML

Chances of _tpl_log_store_get_events_for_date returning NULL due to an
invalid XML file are lower because we ask libxml2 (since commit
734af80) to recover from errors if possible. Even then, it doesn't
hurt to defend against this eventuality.

Thanks to Daniel Thompson for catching this!

https://bugs.freedesktop.org/show_bug.cgi?id=89595

telepathy-logger/log-iter-xml.c

@@ -81,8 +81,12 @@ tpl_log_iter_xml_get_events (TplLogIter *iter,
               priv->account, priv->target, priv->type_mask,
               (GDate *) priv->next_date->data);
 
-          priv->next_event = g_list_last (priv->events);
           priv->next_date = g_list_previous (priv->next_date);
+
+          if (priv->events == NULL)
+            continue;
+
+          priv->next_event = g_list_last (priv->events);
         }
 
       event = TPL_EVENT (priv->next_event->data);