[muc] Prevent duplicate processing of mediated invitations

MUC mediated invitations usually have the form

<message
    from='coven@chat.shakespeare.lit'
    id='nzd143v8'
    to='hecate@shakespeare.lit'>
  <x xmlns='http://jabber.org/protocol/muc#user'>
    <invite from='crone1@shakespeare.lit/desktop'>
      <reason>
        Hey Hecate, this is the place for all good witches!
      </reason>
    </invite>
    <password>cauldronburn</password>
  </x>
</message>
(source: XEP-0045 Example 57.)

However, previous versions of XEP-0045 specified an additional <x
xmlns='jabber❌conference'> element to be included (see
implementation note in XEP-0045). Therefore, a legacy implementation
may emit a mediated invitations in the form of

<message
    from="smack-inttest-mediated-invite-from-8ta77-hw9igz@conference.example.org"
	to="smack-inttest-two-8ta77@example.org">
  <x xmlns="http://jabber.org/protocol/muc#user">
    <invite from="smack-inttest-one-8ta77@example.org"/>
  </x>
  <x xmlns="jabber❌conference" jid="smack-inttest-mediated-invite-from-8ta77-hw9igz@conference.example.org"/>
</message>

Unfortunately, this matches
MultiUserChatManager.DIRECT_INVITATION_FILTER because
GroupChatInvitation matches <x xmlns="jabber❌conference"/>. However
the message is not a direct invitation but a mediated one. Besides
this invoking the wrong listeners (direct vs. medidated) the value for
'inviter' that's used to invoke that listener will be false.

To fix this, extend DIRECT_INVITATION_FILTER with
NotFilter.of(MUCUser.class) to avoid matching those legacy mediated
invitations.

Fixes SMACK-943

Co-authored-by: Florian Schmaus <flo@geekplace.eu>

smack-extensions/src/main/java/org/jivesoftware/smackx/muc/MultiUserChatManager.java

@@ -148,6 +148,7 @@ public final class MultiUserChatManager extends Manager {
     private static final StanzaFilter DIRECT_INVITATION_FILTER =
         new AndFilter(StanzaTypeFilter.MESSAGE,
                       new ExtensionElementFilter<GroupChatInvitation>(GroupChatInvitation.class),
+                      NotFilter.of(MUCUser.class),
                       new NotFilter(MessageTypeFilter.ERROR));
 
     private static final ExpirationCache<DomainBareJid, DiscoverInfo> KNOWN_MUC_SERVICES = new ExpirationCache<>(