TB when someone you are talking to in PM change his nick

[Jim++]

and nick is not modified in PM window

Traceback (most recent call last):
  File "/home/jim/gajim/gajim-propre-svn/src/common/xmpp/idlequeue.py", line 132, in process_events
    obj.pollin()
  File "/home/jim/gajim/gajim-propre-svn/src/common/xmpp/transports_nb.py", line 361, in pollin
    self._do_receive() 
  File "/home/jim/gajim/gajim-propre-svn/src/common/xmpp/transports_nb.py", line 500, in _do_receive
    self.on_receive(received)
  File "/home/jim/gajim/gajim-propre-svn/src/common/xmpp/dispatcher_nb.py", line 355, in dispatch
    handler['func'](session,stanza)
  File "./common/connection_handlers.py", line 2004, in _presenceCB
    avatar_sha))
  File "./common/connection.py", line 175, in dispatch
    self.put_event((event, data))
  File "./common/connection.py", line 171, in put_event
    gajim.handlers[ev[0]](self.name, ev[1])
  File "./gajim.py", line 1203, in handle_event_gc_notify
    c = gajim.contacts.contact_from_gc_contact(gc_c)
  File "/home/jim/gajim/gajim-propre-svn/src/common/contacts.py", line 521, in contact_from_gc_contact
    jid = gc_contact.get_full_jid()
AttributeError: 'NoneType' object has no attribute 'get_full_jid'

[asterix]

(In [9416]) update GC_Contact instance when we change our nickname in a room. fixes #3784