TB on connect

[steve-e]

While connecting I decided to become invisible instead of available.

Traceback (most recent call last):
  File "/home/steve/projekte/python/gajim/src/roster_window.py", line 3257, in change_status
    change(None, account, status)
  File "/home/steve/projekte/python/gajim/src/roster_window.py", line 3247, in change
    self.send_status(account, status, message)
  File "/home/steve/projekte/python/gajim/src/roster_window.py", line 3204, in send_status
    gajim.connections[account].change_status(status, txt, auto)
  File "/home/steve/projekte/python/gajim/src/common/connection.py", line 805, in change_status
    self.send_invisible_presence(msg, signed)
  File "/home/steve/projekte/python/gajim/src/common/connection.py", line 652, in send_invisible_presence
    self.connection.SendAndCallForResponse(iq, self._continue_invisible,
AttributeError: 'NoneType' object has no attribute 'SendAndCallForResponse'

[asterix]

(In [cb0b263418e6f93ad374752cf4739a006764302f]) prevent a traceback. fixes #3202