jabber: Fix chat joins when ext_jid is provided for your own user
This is the presence needed to finalize a join to the channel, the
'from' field is our own user in the context of that channel:
<presence to="dx@server/resource" from="channel@…/dx">
<x xmlns="http://jabber.org/protocol/muc#user">
<status code="110"/>
<item jid="~dx@198.51.100.1" affiliation="none" role="participant"/>
</x>
</presence>
Bitlbee takes the stuff to <item jid="..."> as the "real" jid, as it
does for non-anonymous MUCs. It sets that to ext_jid, and uses it as the
handle internally.
In this particular case, that real jid is nonsense for us, so
imcb_chat_add_buddy() interprets that as yet another person joining the
channel, instead of the expected self-join.
This fixes it by checking if the buddy is our own user, and ignoring the
provided ext_jid in those cases.
Thanks to ivucica for reporting this, who was using Biboumi (a gateway
from XMPP to IRC)
