Fix NoneType object is not callable in CTCBeamDecoder

[farisalasmary]

I always get this error when I use keyboard interruption

KeyboardInterrupt:                                                                                                                                                                     
Exception ignored in: <function CTCBeamDecoder.__del__ at 0x7fd1389638b0>                                                                                                              
Traceback (most recent call last):
  File "/opt/conda/lib/python3.8/site-packages/ctcdecode/__init__.py", line 140, in __del__ 
TypeError: 'NoneType' object is not callable  

This commit aims to solve this issue