Fix shutdown, when _tokenmanager isn't even set

[Finesim97]

Hi, I sometimes get this exception during shutdown:

WARNING:coap:Aborting connection: Server shutdown
ERROR:asyncio:Exception in callback _SelectorSocketTransport._call_connection_lost(None)
handle: <Handle _SelectorSocketTransport._call_connection_lost(None)>
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/asyncio/events.py", line 81, in _run
    self._context.run(self._callback, *self._args)
  File "/usr/local/lib/python3.8/asyncio/selector_events.py", line 970, in _call_connection_lost
    super()._call_connection_lost(exc)
  File "/usr/local/lib/python3.8/asyncio/selector_events.py", line 728, in _call_connection_lost
    self._protocol.connection_lost(exc)
  File "/usr/local/lib/python3.8/site-packages/aiocoap/transports/tcp.py", line 167, in connection_lost
    self._ctx._dispatch_error(self, exc)
  File "/usr/local/lib/python3.8/site-packages/aiocoap/transports/tcp.py", line 259, in _dispatch_error
    if self._tokenmanager is None:
AttributeError: 'TCPClient' object has no attribute '_tokenmanager'

I think this fixes the issue. Have a nice day