Fix exception on reconnect

This is a regression from 5fa56684

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/gi/overrides/GLib.py", line 662, in <lambda>
    func_fdtransform = lambda _, cond, *data: callback(channel, cond, *data)
  File "/home/andrey/progs/python-nbxmpp.git/nbxmpp/idlequeue.py", line 545, in _process_events
    return IdleQueue._process_events(self, fd, flags)
  File "/home/andrey/progs/python-nbxmpp.git/nbxmpp/idlequeue.py", line 407, in _process_events
    obj.pollend()
  File "/home/andrey/progs/python-nbxmpp.git/nbxmpp/transports_nb.py", line 477, in pollend
    (self.server, self.port))
  File "/home/andrey/progs/python-nbxmpp.git/nbxmpp/transports_nb.py", line 216, in _on_connect_failure
    self.on_connect_failure(err_message=err_message)
  File "/home/andrey/progs/python-nbxmpp.git/nbxmpp/client_nb.py", line 325, in _try_next_ip
    self.disconnect(msg)
  File "/home/andrey/progs/python-nbxmpp.git/nbxmpp/client_nb.py", line 174, in disconnect
    self.on_connect_failure()
  File "/home/andrey/progs/gajim.git/gajim/common/connection.py", line 1035, in _connect_to_next_host
    self._connect_to_next_host(retry)
  File "/home/andrey/progs/gajim.git/gajim/common/connection.py", line 1005, in _connect_to_next_host
    self.disconnectedReconnCB()
  File "/home/andrey/progs/gajim.git/gajim/common/connection.py", line 673, in disconnectedReconnCB
    if not self.connection.resume_supported:
AttributeError: 'NoneType' object has no attribute 'resume_supported'
5 jobs for fix-exception-on-reconnect in 3 minutes and 46 seconds (queued for 2 minutes and 10 seconds)
latest
Status Job ID Name Coverage
  Test
passed #8773
run-appdata

00:00:05

passed #8771
run-mypy

00:00:23

passed #8772
run-pylint

00:03:02

passed #8770
run-test

00:00:06

 
  Build
passed #8774
run-build

00:00:08