asif sometimes crashes when someone parts a channel on freenode

[ddevault]

[2017-12-29 21:08:34,896] DEBUG: bot.py:_get_message(402): -> :[email protected] PART #sway                                                              
[2017-12-29 21:08:34,896] DEBUG: bot.py:run(431): Calling command handler CommandHandler(args=('PART',), rest=None, handler=<bound method Client._on_part of <asif.bot.Client object at 0x7f0792028198>>) with input
 IrcMessage(prefix='[email protected]', args=('PART', '#sway'), rest=None)                                                               
[2017-12-29 21:08:34,896] DEBUG: bot.py:__init__(32): Created <User [email protected]>                          
Traceback (most recent call last):                                                              
  File "./bot.py", line 46, in <module>                                                                                                                       
    loop.run_until_complete(bot.run())                                                                                                                                
  File "/usr/lib/python3.6/asyncio/base_events.py", line 467, in run_until_complete             
    return future.result()                                                                                                                                             
  File "/usr/lib/python3.6/site-packages/asif-0.0.1-py3.6.egg/asif/bot.py", line 432, in run                                                                              
  File "/usr/lib/python3.6/site-packages/asif-0.0.1-py3.6.egg/asif/bot.py", line 617, in _on_part                                                                       
KeyError: <User [email protected]>