text: Title: Python 3.7.3, gensim 3.8.1: UnboundLocalError: local variable 'doc_no2' referenced before assignment
  Body: Client code:
  `    model = LogEntropyModel(corpus=data_corpus, normalize=True)
  `
  Referenced code:
  https://github.com/RaRe-Technologies/gensim/blob/44ea7931c916349821aa1c717fbf7e90fb138297/gensim/models/logentropy_model.py#L115
  
  Exception thrown:
  
  ```
    File "/anaconda3/lib/python3.7/site-packages/gensim/models/logentropy_model.py", line 76, in __init__
      self.initialize(corpus)
    File "/anaconda3/lib/python3.7/site-packages/gensim/models/logentropy_model.py", line 115, in initialize
      if doc_no2 != doc_no:
  UnboundLocalError: local variable 'doc_no2' referenced before assignment
  
  ```