Don't crash when 'from' is undefined

c2s_filter_send hook may pass a stanza with undefined from/to (due to
legacy auth for example). Work around this problem.

Fixes #2036

src/mod_client_state.erl

@@ -265,7 +265,10 @@ filter_other({Stanza, #{jid := JID} = C2SState} = Acc) when ?is_stanza(Stanza) -
 	    Acc;
 	_ ->
 	    ?DEBUG("Won't add stanza for ~s to CSI queue", [jid:encode(JID)]),
-	    From = xmpp:get_from(Stanza),
+	    From = case xmpp:get_from(Stanza) of
+		       undefined -> JID;
+		       F -> F
+		   end,
 	    C2SState1 = dequeue_sender(From, C2SState),
 	    {Stanza, C2SState1}
     end;