Prevent accessing list index when no device is available

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

slixmpp_omemo/__init__.py

@@ -302,6 +302,8 @@ class XEP_0384(BasePlugin):
         self._store_device_ids(bare_jid, items)
 
         items = list(items)
+        device_ids = []
+        if items:
             device_ids = [int(d['id']) for d in items[0]['devices']]
 
         if bare_jid == self.xmpp.boundjid.bare and \